This is a fork to show my personal changes.
Some may rely on my CSS changes, so they are meant to get inspiration to do something differently, not to be used as is (though it may work, with luck).
Below is the normal README.md from the ordinary project (some things are not relevant to my branch).

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Custom JS scripts for Firefox 60+ and Thunderbird 68+

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

[outdated] Method 1 - files in profile folder only (Fx60-71)

M1 is based on this project by nuchi: https://github.com/nuchi/firefox-quantum-userchromejs
M1 stopped working in Firefox 72! XBL support got dropped.

From this projects \method 1\ folder copy userChrome folder and userChrome.css file to \ PROFILENAME \chrome\ or add code from userChrome.css file to an existing userChrome.css file.

Edit userChrome\userChrome.xml file to add custom scripts.

Method 2 - files in install and profile folders

M2 is based on this project by ardiman: https://github.com/ardiman/userChrome.js
M2 is also based on the modified files by Endor8: https://github.com/Endor8/userChrome.js/

From this projects \method 2\profile\ folder copy userChrome folder and userChrome.js file to \ PROFILENAME \chrome\ folder.

From this projects \method 2\firefox\ folder copy defaults folder and config.js file to 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".

[outdated] Method 3 - files in install and profile folders (Fx60-91.4)

M3 is based on this project by xiaoxiaoflood: https://github.com/xiaoxiaoflood/firefox-scripts (up-to-date / works with Fx96+)

From this projects \method 3\profile\ folder copy utils folder and userChrome.uc.js file to \ PROFILENAME \chrome\ folder.

From this projects \method 3\firefox\ folder copy defaults folder and config.js file to Firefox/Thunderbird main directory (where the Firefox/Thunderbird executable is).

Edit userChrome.uc.js file to add custom scripts or delete userChrome.uc.js file and add scripts directly into /chrome/ folder.

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
More info about startup cache removal (in English [Google translation]): https://translate.googleusercontent.com/translate_c?act=url&depth=1&ie=UTF8&prev=_t&rurl=translate.google.com&sl=auto&sp=nmt4&tl=en&u=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