Hi, that is the best case scenario: that a new installation of Firefox could log in to your Firefox Account and get your saved data merged with the default data in the new installation.

We do see a number of reports from users that Sync didn't work as a backup, perhaps because it was designed to synchronize data between two installations of Firefox connected at the same time?

If in doubt, you can make backups of your Firefox data. Not all of the files are crucially important, but it's certainly simplest to back up the entire profile folder. Because it contains history and possibly saved logins, the data should be stored securely. More info: Back up and restore information in Firefox profiles

You can backup certain files in the current profile folder with Firefox closed.

• 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)

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data