The Firefox web browser stores information about its user settings and state in a "profile" directory for each user. On a Mac OS X system (now macOS), such as my MacBook Pro laptop, you can have Firefox open a Finder window showing you the relevant profile folder or you can find it from a command line interface (CLI), i.e., a shell prompt, by opening a Terminal window (you can find the Terminal program in the Applications/Utilities folder). For the latter method, you can see the location of the profile directory by issing the command
ls -l ~/Library/Application\ Support/Firefox/Profiles.
(you need to put a backslash before the space in the directory path)
$ ls -l ~/Library/Application\ Support/Firefox/Profiles total 0 drwx------ 53 jasmith1 ABC\Domain Users 1802 Jan 23 11:56 bgq13udo.default $
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