One of the reasons I use Mozilla Firefox is its huge universe of extensions, scripts, styles, and other add-ons which can make it behave the way you want it to. I've customized my Firefox installation pretty extensively.

This list is current as of Firefox 62.0.

Extensions

Extension list was very old and pre-Firefox Quantum; removed.

Preferences

user.js settings I use:

Setting

Value

Rationale

browser.backspace_action

1

Bind backspace to scroll up a page

browser.display.normal_lineheight_calc_control

1

Not exactly sure what this does, but fixes some line-height/font rendering problems

browser.fullscreen.autohide

false

Prevent tab/URL bar from auto-hiding in full-screen mode

browser.safebrowsing.enabled

false

Disable Safe Browsing and malware checking (don't care for either on Linux)

browser.safebrowsing.malware.enabled

false

image.animation_mode

once

Animated GIFs only animate once

intl.charset.default

UTF-8

Default character set

layout.css.dpi

0

Use system-reported DPI, always (normal behavior may sometimes use 96 DPI)

layout.spellcheckDefault

2

Enable spell checking for both INPUT and TEXTAREA fields

network.http.sendRefererHeader

1

Only send HTTP referer header for links clicked on, not for images/media/etc. Breaks some Websites.

network.protocol-handler.external.mailto

true

network.proxy.socks_remote_dns

true

Send DNS requests through proxies

privacy.trackingprotection.enabled

true

Enable Firefox's Tracking Protection

security.dialog_enable_delay

0

No wait when prompted to install unsigned extensions

services.sync.addons.ignoreUserEnabledChanges

true

Do not sync the enabled/disabled state of Add-ons

ui.caretBlinkTime

0

Disable cursor blinking

ui.allow_platform_file_picker

false

Don't use the horrid GTK file picker

browser.urlbar.formatting.enabled

false

Don't just highlight just the domain in the URL bar

browser.urlbar.trimURLs

false

Don't strip http:// from URLs

layout.css.devPixelsPerPx

2

Double CSS pixels, for retina displays

widget.content.gtk-theme-override

Breeze:light

When using a dark GTK theme, fix colors for widgets in Firefox

xpinstall.signatures.required

false

Enable installation of unsigned extensions (needed for addons not available on addons.mozilla.org)

security.insecure_connection_text.enabled

true

Show icon/text that non-HTTPS sites are insecure

security.insecure_connection_icon.enabled

true

general.smoothScroll.mouseWheel

false

Disable smooth scrolling for mouse input (should use XInput2 instead)

Experimental settings:

network.predictor.enable-prefetch

true

None in use.

Chrome

Contents of my userChrome.css:

   1 /* Font preferences for AwesomeBar dropdown
   2  * 
   3  * Adjusts font size, underlines URLs, and corrects colors
   4  */
   5 .ac-comment
   6 {
   7   font-size: 100% !important;
   8 }
   9 
  10 .ac-url-text
  11 {
  12   color: #306090 !important;
  13   text-decoration: underline !important;
  14 }
  15 
  16 .ac-url-text[selected="true"]
  17 {
  18   color: white !important;
  19 }
  20 
  21 /* Use monospaced font for AwesomeBar */
  22 #urlbar
  23 {
  24   font-family: monospace !important;
  25   font-size: 14px !important;
  26 }
  27 
  28 /* Combines Stop/Reload buttons, such that only one is shown at a time
  29  *
  30  * Note: May require button reordering (e.g reverse order of stop/reload
  31  * buttons) if the below does not work
  32  *
  33  * Derived from user style: http://userstyles.org/styles/10
  34  * Also, see bug 343396: https://bugzilla.mozilla.org/show_bug.cgi?query_format=specific&order=relevance+desc&bug_status=__open__&id=343396
  35  */
  36 #stop-button[disabled="true"] { display:none; } 
  37 #stop-button:not([disabled]) + #reload-button { display:none; }
  38 
  39 /* Italicize unread tabs
  40  *
  41  * Derived from user style: http://userstyles.org/styles/8247
  42  */
  43 #content tab:not([selected]) { font-style: italic !important; }
  44 
  45 /* Display keyword field in Bookmark dialog
  46  *
  47  * Derived from user style: http://userstyles.org/styles/8250
  48  */
  49 #editBMPanel_keywordRow { visibility: visible; }
  50 
  51 /* Hide search and go buttons */
  52 .search-go-button,
  53 .search-go-button-stack,
  54 #go-button
  55 {
  56   display: none !important;
  57 }

Android

Using Roboto or another font instead of Open Sans

  1. Go to about:config
  2. Set to "Roboto", "Nokia Pure Text", or "Alte Haas Grotesk"
    • font.name.sans-serif.x-unicode
    • font.name.sans-serif.x-user-def
    • font.name.sans-serif.x-western

Font size adjustments

  1. Go to about:config
  2. Set font.minimum-size.x-unicode and font.minimum-size.x-western to 14
  3. Set font.size.inflation.minTwips to "140"

Cache location

On devices w/ 8–16 GB memory, store Firefox's cache on the external SD card:

  1. Go to about:config
  2. Set browser.cache.disk.parent_directory to one of:
    • /storage/sdcard1/Android/data/org.mozilla.fennec/files/cache/
    • /storage/sdcard1/Android/data/org.mozilla.firefox_beta/files/cache/
    • /storage/sdcard1/Android/data/org.mozilla.firefox/files/cache/

Enabling off-main-thread compositing

Set to true:

You can also set:

to display a FPS counter in the top left corner. If you see the counter, than OMTC and layer acceleration are working.

722012 – [Meta] Implement OMTC on Linux tracks whether OMTC has been enabled on Linux.

Fiddling

To tinker w/ browser styles/events/etc, navigate to chrome://browser/content/browser.xul and launch the built-in developer tools.

External tools

https://gist.github.com/f1u77y/715f0dd03fc18039a6549de557370b97: Exports styles from Stylish extension (stored in stylish.sqlite) into a JSON file that can be loaded into Stylus.

Interesting reading