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Instructions

Video running installation steps below: https://youtu.be/_4fdUdp3G4o

Download this zip file and extract its content to Firefox installation folder (usually C:\Program Files\Mozilla Firefox). For Linux paths, read this. And this for macOS paths.

Click Firefox menu button (☰) -> Help -> More troubleshooting information or simply open about:support. Find "Profile Folder", then click "Open folder". In there, create a new folder called chrome.

Download one of the files below and extract its content in chrome folder.

utils → I'm only interested in scripts

utils → I'm only interested in extensions

utils → I'm interested in both scripts and extensions

Now, if you're only interested in extensions, you can skip to step 6.

Save the desired userChromeJS scripts into chrome. Read below the description of some of them.

If you want a button to manage your scripts, including the ability to disable/enable scripts without needing to restart1 Firefox or Thunderbird, save rebuild_userChrome.uc.js into chrome.

Open about:support and click "Clear startup cache…" to force Firefox to load userChromeJS on the next startup.

Restart Firefox.