Backup

The easiest, and complete, way to backup is to copy your entire profile directory when Firefox is closed. Just backing up prefs.js file is not enough, and not recommended by this wiki.[^1]

To find your profile directory, go to about:support and about the 10th item listed is Profile Folder with an Open Folder button.

When copying your profile directory, make sure Firefox is closed. Rename your copied folder to something meaningful, and keep the original name (for an easy rollback). For example, if the profile folder is called jiu9k8gm.default, a backup copy might be called jiu9k8gm.default-backup or jiu9k8gm.default-pre-arkenfox-userjs.

Be aware, that this is your entire profile. So rolling back will not keep any new cookies, saved passwords, bookmarks, extension changes and the like.

Warning

Critical content demanding immediate user attention due to potential risks.

Always make a backup!

[^1]: prefs.js is a runtime file that includes more than just your pref changes, and backing up prefs.js does not cover all the things a user.js can alter.