If I made a firefox account, will all my history be saved on this account or just the history after making this account? I am asking this because I am going to perform a clean OS installation and I need absolutely all my history.
Firefox account is for sync, it doesn't work well as a backup. You need a full backup.
How do I back it up? From what I can see it only backs up bookmarked history.
There is no backup of the browsing history, so your only option is to backup the places.sqlite database (bookmarks and history).
You can copy certain files with Firefox closed to the current profile folder to transfer or recover personal data. Note that best is to avoid copying a full profile folder.
You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).
- Help -> More Troubleshooting Information -> Profile Folder/Directory:
Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
- bookmarks and history: places.sqlite
- favicons: favicons.sqlite
- bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
- cookies.sqlite for the Cookies
- formhistory.sqlite for saved autocomplete Form Data
- logins.json (encrypted logins;32+) and key4.db (decryption key;58+) for Passwords saved in the Password Manager
key3.db support ended in 73+; to use key3.db in 58-72, make sure to remove key4.db
- cert9.db (58+) for (intermediate) certificates stored in the Certificate Manager
- persdict.dat for words added to the spell checker dictionary
- permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
- sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)