JMM’s

about:config Notes and Settings

for Firefox

browser.tabs.closeWindowWithLastTab
I set this to “false” so closing a tab doesn’t force the browser window to close.
browser.urlbar.filter.javascript
I’ve set this to “false”, but I’m not sure if it still does anything. It’s supposed to allow you to type “javascript:” URLs, which are normally disabled for security reasons (i.e. naïve users being tricked into entering it).
browser.urlbar.showSearchSuggestionsFirst
I’ve set this to “false”.
browser.urlbar.suggest.calculator
“true”
browser.urlbar.suggest.quicksuggest.sponsored
“false”
browser.urlbar.suggest.searches
“false”
browser.urlbar.suggest.topsites
“false”
devtools.editor.keymap
“emacs”
extensions.pocket.enabled
“false”
identity.fxaccounts.enabled
Set to “false” to disable Firefox sync/accounts. Might be useful if you want to prevent accidentally sending your browsing history, passwords, and other data elsewhere.
layout.css.grid-template-masonry-value.enabled
“true”. Just to test out this CSS grid value.
browser.eme.ui.enabled
media.eme.enabled
“false”. Disable DRM. See https://support.mozilla.org/en-US/questions/1388341.
I’ve also set “media.gmp-widevinecdm.enabled = false” and “media.gmp-widevinecdm.visible = false”.
toolkit.legacyUserProfileCustomizations.stylesheets
“true”
Allows you to style your Firefox chrome layout with CSS.
ui.prefersReducedMotion
Set it to “1” to set your CSS “prefers-reduced-motion”. Can also set it to “0”.
browser.startup.homepage
“about:blank”
browser.privatebrowsing.promoEnabled
Set to “false”. I think this blocked a promo for Mozilla VPN (which, guess what, I already use).
browser.newtabpage.activity-stream.showSponsoredTopSites
“false”
browser.compactmode.show
Set to “true”. I forget why.
dom.no_unknown_protocol_error.enabled
Set to “false” to show an error when Firefox doesn’t know what to do with a URI scheme. Like this magnet link for GPL-v3
This unknown protocol page isn’t enabled by default anymore unless you switch this config. Screenshot taken on .

This didn’t seem to work on Firefox Nightly on Android 121.0a1 on 2023-11-13.

browser.urlbar.suggest.addons
Set to “false” to remove extension addons like the one below.
Suggestions for extensions in the URL bar. Screenshot taken 2023-12-12.
Same as above, but with suggestion removed.
image.jxl.enabled
In recent versions (possibly only Firefox Nightly) this enables JPEG-XL support. I have not yet (as of 2024-02-05) tried it.
network.disable-localhost-when-offline
File > Work Offline isn’t actually offline unless you set this to true.
print.print_headerleft
print.print_headercenter
print.print_headerright
print.print_footerleft
print.print_footercenter
print.print_footerright
Changes the headers/footers in printing. See this comment by jscher2000.
&T
Title
&U
URL
&PT
Page number of total
&P
Page number
&D
Date/Time
dom.private-attribution.submission.enabled
Set to “false” to disable Privacy-Preserving Attribution. Documented by kivikakk in lobste.rs.
javascript.enabled
You can toggle this to “false” to turn off JavaScript.

Saving preferences

prefs.js inside the user profile suggests adding preferences to user.js (e.g. ~/.mozilla/firefox/xxxxxxxx.xxxxxxx/user.js). Haven’t tried it yet.