Custom JS scripts for Firefox and Thunderbird

The Wiki contains a feature list (with screenshots).

Unlock custom CSS usage in Firefox 69 and newer

about:config > toolkit.legacyUserProfileCustomizations.stylesheets > true

Unlock custom CSS usage in Thunderbird 69 and newer

Settings/Options > Advanced > General > Config Editor...
toolkit.legacyUserProfileCustomizations.stylesheets > true

Script Loader

Based on this project by ardiman: https://github.com/ardiman/userChrome.js
Additionally based on the modified files by Endor8: https://github.com/Endor8/userChrome.js/

Copy from this projects \script_loader\profile\ folder userChrome folder and userChrome.js file to \ PROFILENAME \chrome\ folder.

Copy from this projects \script_loader\firefox\ folder defaults folder and config.js file into Firefox main directory (where the Firefox executable is; or on macOS, inside the main executable, at /Applications/Firefox.app/Contents/Resources/).

Edit userChrome.js file to add custom scripts!

With Firefox 62+ and Thunderbird 68+ an additional preference pref("general.config.sandbox_enabled", false); has to be set inside config-prefs.js file. This is considered less secure by Mozilla and is only a temporary workaround, but at the moment it is the only way to run custom scripts using "method 2".

Script/startup cache must be deleted after every change!

Where to find Firefox startupCache folder?
about:profiles > Local Directory > Open Folder, close Firefox and delete all files in startupCache folder.

This is not the same 'profile' folder custom scripts and styles are stored in!
Where to find Thunderbird startupCache folder?
Windows
C:\Users\ USERNAME \AppData\Local\Mozilla\Thunderbird\Profiles\ PROFILE FOLDER NAME \
Linux/macOS
Search for startupCache folder on your hard drive.

More info about startup cache removal (in German): https://github.com/ardiman/userChrome.js/wiki/Skriptcache

Where to find Firefox folder with main executable?

about:support > Application Binary (Path on your system)

Where to find Firefox profile folder?

about:profiles > Root Directory > Open Folder or
about:support > Profile Folder > Open Folder

Where to find Thunderbird profile folder?

Windows
C:\Users\ USERNAME \AppData\Roaming\Mozilla\Thunderbird\Profiles\ PROFILE FOLDER NAME \
Hidden files must be visible to see AppData folder. Alternatively open %APPDATA%\Mozilla\Firefox\Profiles\ from explorers location bar.

Linux
/home/ username /.mozilla/thunderbird/ profile folder name /
Hidden files must be visible to see .mozilla folder.

Mac OS X
~\Library\Mozilla\Thunderbird\Profiles\ PROFILE FOLDER NAME \ or
~\Library\Application Support\Mozilla\Thunderbird\Profiles\ PROFILE FOLDER NAME \
\Users\ USERNAME \Library\Application\Support\Thunderbird\Profiles\

Script collections

Script collection by ardiman: https://github.com/ardiman/userChrome.js
Script collection by Endor8: https://github.com/Endor8/userChrome.js
Script collection by xiaoxiaoflood: https://github.com/xiaoxiaoflood/firefox-scripts/tree/master/chrome