“Here be dragons,” reads the ominous disclaimer when you type
about:config into Firefox’s URL bar, warning you that tweaking things in this area is largely experimental and can cause instability to your browser.
Sounds exciting, right? And even though it sounds a little scary, the fact is that you will almost certainly be okay when you start playing around in this area and can actually use the features here to improve and speed up your browser. These are Make Tech Easier’s favorite Firefox about:config tricks, freshly updated for Firefox Quantum.
How to Access “about:config”
about:config into Firefox’s address bar.
You will be shown that dragon-themed warning page. Click the “I accept the risk!” button, and you’ll arrive on the about:config homepage.
You’ll see a long list of configuration entries. If you’re looking for a specific one, type its name into the “Search” bar above the list.
To toggle different features on and off, simply double-click their entries under the “Value” column to switch between “true” and “false.” In some cases the value field will have a number instead. In this case change the number to the one that corresponds to what you want. (Don’t mess around with these numbers until you know what you’re doing!)
Now, let’s get tweaking.
Note: to undo specific changes you make in about:config, just right-click the entry you want to revert and click “Reset.”
1. Disable Unnecessary Animations
Animations in Firefox Quantum are not a bad thing, but if you have an old PC where every MB of RAM counts or simply don’t need these animated flourishes, you can disable them by going to
toolkit.cosmeticAnimations.enabled and setting the value to “false.”
Default value: true
Modified value: false
2. Change Minimum Tab Width
It will take a more sharp-eyed Firefox user to notice this adjustment Mozilla made for Firefox Quantum. The default tab width is now just 76 pixels, whereas before it was 100. To adjust this, go to
Default value: 76
Modified value: 100 if you want the same tab width as in older versions of Firefox, but really you can make this whatever you like. Just don’t go overboard!
3. Reduce Session History Cache, Save RAM
If you’re using an older machine, then even the typically speedy Firefox may slow down your PC with the default settings. This could be in part because of how it stores Web pages in its short-term memory (or RAM), which you can access using the Back and Forward buttons.
browser.sessionhistory.max_total_viewers affects how many pages Firefox stores in such a way that they load super fast.
Default value: – 1 (adaptable)
Modified value: any number, reflecting the number of pages you want to store. (We recommend less than 4 if your PC is struggling for speed, while those with 4GB plus can go for 8 or more.)
browser.sessionhistory.max_entries affects how many pages each tab stores in its Back/Forward history altogether.
Default value: 50
Modified value:If your PC is struggling, lower this to 25, check if it helps, then adjust accordingly.
4. Turn Off Preview Thumbnails
When you press Ctrl + Tab in Firefox, it will toggle between the different tabs you have open in your browser. This brings up slick little thumbnails showing each page you have open. It looks good but can be resource-consuming on weaker PCs, so you can turn it off by changing
Default value: true
Modified value: false – automatically switches pages on Ctrl+Tab instead of showing you thumbnails.
5. Disable Extension Compatibility Checks
Compatibility checks. Who needs ’em, right? Actually they’re pretty handy as a general reference of which extensions will work with your version of Firefox and which won’t, but Firefox doesn’t always get it right. If you want to see whether an extension that Firefox claims is incompatible may actually work, do the following:
- Right-click anywhere on the about:config page, then click “New -> Boolean”
extensions.checkCompatibilityinto the box, click OK, then select “false” and click OK again.
- This preference will now exist in your list, and you can disable it at any time by right-clicking it and clicking “Reset.”
6. Change Firefox Download Location
By default, Firefox downloads go to the Windows “Downloads” folder, but you can change this by tweaking
Default value: 1
- 0 – Saves all downloads to the desktop
- 2 – Saves to the same location as the previous download
7. Get Asked Where You Want Each Download Saved
If you want to have more direct control over your downloads and decide what directory you want each one to be saved in, change the preference
browser.download.useDownloadDir to “false.”
Default value: true
Modified value: false – prompts you where to save each download
8. Open New Tab for Search Box Results
By default, the things you search for in the Firefox search box open in the current tab. To open in a new tab instead, you’ll need to modify
Default value: false – opens search results in current tab
Modified value: true – opens search results in new tab
9. New Tabs Page
Firefox’s New Tabs page organizes all the sites you’ve bookmarked and are most likely to visit in a convenient grid. Best of all, you can tweak how big this grid is, so while it uses 3×3 thumbnails by default, you can change it thanks to the
Default value: 3 in “rows,” 5 in “columns”
Modified values: Whatever number you like!
10. Adjust the Smart Location Bar’s Number of Suggestions
In Firefox when you start typing in the location (or URL) bar, a dropdown list of suggested sites will be shown. If you want it to show more (or less) than ten suggestions, you can adjust the
browser.urlbar.maxRichResults keys and get it to show the number you want.
Default value: 10
Modified value: Set to your desired number of suggestions. If you want to disable it altogether, set it to -1.
11. Adjust the Session Restore Saving Frequency
Firefox saves your session every fifteen seconds by default, but you can change the value of
browser.sessionstore.interval so that Firefox will save the session at a longer interval.
Default: 15000 (in msecs, equivalent to fifteen seconds)
Modified value: Set it to your desired value. 1000 means one sec, and 60000 means one minute
12. Extend Scripts’ Execution Time
In Firefox a script is only given ten seconds to respond, after which it will issue an unresponsive script warning. If you are stuck on a slow network connection, you might want to increase the script execution time via
dom.max_script_run_time to cut down on the frequency of the no-script warning.
Default value: 10 (in secs)
Modified value: 20, or any value greater than 10
- 0 – open all links the way you have Firefox handle new windows
- 1 – do not open any new windows
14. Enable Spell-Checking in All Text Fields
The default spell-checking function only checks for multi-line text boxes. You can change the option in
layout.spellcheckDefault to get it to spell-check for single line text boxes as well.
Default value: 1 (spellcheck for multi-line text boxes only)
- 0 – disable spellcheck
- 2 – enable spell-check for all text boxes
15. Lower Memory Usage When Minimized
This tweak is mainly for Windows users. When you minimize Firefox, it will send Firefox to your virtual memory and free up your physical memory for other programs to use. Firefox will reduce its physical memory usage, when minimized, to approximately 10MB (give or take some), and when you maximize Firefox it will take back the memory that it needs.
The preference name does not exist and needs to be created.
Right-click on the background and select “New -> Boolean.”
Enter the name when prompted:
Enter the value: True
16. Increase/Decrease the Amount of Disk Cache
When a page is loaded, Firefox will cache it into the hard disk so that it doesn’t need to be downloaded again the next time it is loaded. The bigger the storage size you cater for Firefox, the more pages it can cache.
Before you increase the disk cache size, make sure that
browser.cache.disk.enable is set to “True.”
Default value: 50000 (in KB)
- 0 – disable disk caching
- Any value lower than 50000 reduces the disk cache
- Any value higher than 50000 increases the disk cache
17. Select All Text When You Click on the URL Bar
In Windows and Mac Firefox highlights all text when you click on the URL bar. In Linux it does not select all the text. Instead, it places the cursor at the insertion point. Regardless of which platform you are using, you can tweak
browser.urlbar.clickSelectsAll to either select all or place the cursor at the insertion point.
- False – place cursor at the insertion point
- True – select all text when you click
18. Same Zoom Level for Every Site
Firefox remembers your zoom preference for each site and sets it to your preferences whenever you load the page. If you want the zoom level to be consistent from site to site, you can toggle the value of
browser.zoom.siteSpecific from “True” to “False.”
Default value: True
Modified value: False (enable same zoom preferences for every site)
19. Setting Your Zoom Limit
If you find that the max/min zoom level is still not sufficient for your viewing, you can change the zoom limit to suit your viewing habits.
Default value: 300 (percent)
Modified value: any value higher than 300
Default value: 30 (percent)
Modified value: any value
20. Configure Your Backspace Button
In Firefox you can set your backspace by getting it to either go back to the previous page or scroll up a page if it’s a scrolling site. Holding Shift as a modifier will go forward a page if the value is set to 0 and scroll down if the value is set to 1.
Default value: 0 – goes back a page
Modified value: 1 – scrolls up a page
21. Increase Offline Cache
If you do not have access to the Internet most of the time, you might want to increase the offline cache so that you can continue to work offline. By default, Firefox caches 500MB of data from supported offline web apps. You can change that value to any amount you like.
Default value: 512000 (in KB)
Modified value: any value higher than 512000 will increase the cache value
22. Disable Delay Time When Installing Add-ons
Every time you install a Firefox add-on, you will have to wait for several seconds before the actual installation starts. To cut down on this waiting time, you can turn the preference
security.dialog_enable_delay off so that the installation will begin immediately.
Default value: 1000 (in msec)
- 0 – starts installation immediately
- any other value (in msec)
23. View Source in Your Favorite Editor
This is very useful for developers who are always using the “view source” function. This tweak allows you to view the source code of a given website in an external editor.
There are two configurations that need to be made:
Default value: False
Modified value: True (enable view source using external text editor)
Default value: blank
Modified value: insert the file path to your editor here
24. Increasing “Save Link As” Timeout Value
When you right-click and select “Save Link As … ” the browser will request the content disposition header from the URL to determine the filename. If the URL does not deliver the header within one second, Firefox will issue a timeout value. This could happen very frequently in a slow network connection environment. To prevent this issue from happening frequently, you can increase the timeout value to reduce the possibility of a timeout by editing
Default value: 4000 (4 seconds)
Modified value: any value higher than 1000 (value is in msec)
25. Autohide Toolbar in Fullscreen Mode
In fullscreen mode the toolbar is set to autohide and appear only when you hover over it with your mouse. If you want, you can choose to have it visible all the time instead by toggling the value of
browser.fullscreen.autohide to “False” to always show the toolbar.
Default value: True (always autohide)
Modified value: False (always show the toolbar)
26. Increase Add-on Search Result
If you go to “Tools -> Add-ons -> Get Add-ons” and perform a search, Firefox will display fifteen matching results. If you want more or less results here, you can adjust
Default value: 15
Modified value: more or less than 15
The Firefox about:config section provides a powerful way for users to fully customize the browser. The above list is definitely not the complete list. There are tons of tricks that you can do. If you have any tricks not listed here, please add them in the comments.
This article was first published in 2008 and was updated in March 2018.