There are no backups (or "restore points) saved by Sync, one copy of your data is what Sync saves. You should have backed up your Firefox data before reinstalling your operating system, considering that you have Firefox installed on a 2nd hard drive and you didn't keep that version up to date. That is a complicating factor depending upon how old that version is.

Don't do anything further without thinking your situation more thoroughly, with our help. Wait for other support contributors to weigh in here and let me think this through a bit more.

Do not open any other versions of Firefox that you may have connected to that same Sync account; more than likely you'll just lose data on the other Firefox versions you have. too.

Sync can't be controlled by the user as far as directional control is concerned. Chances are that the device that was has the earliest or oldest connection date/time to the Sync server is going to receive or delete data to synchronize with what is now on the Sync server. The only control that you have is to dis-connect from the Sync server and figure out a plan of action to minimize your potential loss of data. Do that for now.

Please explain more completely about that older, 2nd version that you found on that PC. Does that drive have its own different operating system (like dual booting Win7 / Win 10) ?

Unless you specifically created a unique Profile for that version of Firefox on that same hard drive, your Profile data for that version is gone. Firefox places the Profile folder (which is where your Firefox data) on the drive where the operating system is located, plus Firefox will use the same Profile for every version of Firefox that is installed on that PC, unless you programmatically specify otherwise.

The only thing I backed up was my tab session via an extension that stored it all in a json format. It was completely my fault for not thinking to back everything else up. Where are the various Firefox settings and such stored? I may be able to restore those files via a program like Recuva if I know where to look.

The Firefox that was on my 2nd drive was approximately verison 43. It is a storage hard drive -- so it doesn't have an operating system. I just didn't realize I had been using an instance of Firefox on my OS drive the whole time until I opened Firefox on my 2nd drive.

First, what about another desktop / laptop device that you were using on your Sync Account (Firefox Account, as it is now known) ? You should be able to recover a bookmarksbackup file from a date prior to this problem. https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them#w_restoring-from-backups You would need to copy the JSON (or jsonlz4 compressed backup) file from that PC to the one you are working on; and then Restore > Use file ... with that file thru the Library window as explained in that support article.

When Firefox is installed it creates a Profile (plus the file that 'controls' the Profiles) in %APPDATA%\Mozilla\Firefox\Profiles\ for user data https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

With Firefox 43 or whatever version on the non-OS 2nd drive your Profile data wouldn't be on that drive - the user data would have been on the OS drive which you overwrote with a fresh OS installation.

But if you really were able to open that older Fx43 version that is sitting on the 2nd drive, chances are it would have created a new Profile on the OS drive automatically, due to not finding any trace of a Firefox Profile file structure on the fresh OS installation. That is assuming that you didn't already install Firefox normally, where Firefox would install Firefox in the default location in Program Files on the OS drive.

Yeah, it can sound confusing, but there are a number of variables involved with any application and the user can many times deviate from the "defaults" accidentally or purposely if they so wish to do. I am just trying to foresee what could have happened. 17 years using Firefox by now, and I have broken my Firefox installations in more ways than the typical user could imagine; sometimes on purpose when 'pushing the limits' to see what that would do, while other times just not fully thinking thru my actions. But always with my Profile backed up - JustinCase.

"The only thing I backed up was my tab session via an extension that stored it all in a json format." Save that file and use it once you're successful with recovering your bookmarks.

Recuva may work to recover your old Profile data files. See this support article to see which files to search for. https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile#w_your-important-data-and-their-files

As far as "restoring" the Profiles files that you are able to recover, see this support article. https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles#w_restoring-a-profile-backup

I wish you success with this.

Thanks for the additional information. I should also note that I don't have another computer that I use Firefox on -- just my home PC and my phone. The question was basically just a "what if" for backing up my data so that I don't have to reconfigure anything next time I use Firefox. Sorry for the confusion.

I don't know if I lost all of my bookmarks -- many newer ones I recognize are still there. The big thing is that they aren't organized like they were before -- they're all over the place -- which is why I assumed that I likely lost some of them.

I shouldn't have asked about where profile information is stored because immediately after posing that question, I looked it up myself. I restored what I could get back from the profiles folder using Recuva, but it wasn't a lot and I don't think any of it is useful or usable. I don't have any of the files from the first of the two links you provided.

So my old profile is effectively gone, and I have *most* of my data (as far as I can tell), but I've definitely lost certain settings, themes, and extensions. In the future, I'll be sure to back up %APPDATA%\Mozilla\Firefox\Profiles.

UPDATE: After logging back into my Firefox Account, my extensions were added back. I didn't kept any of the extension data itself (like my one or two TamperMonkey scripts), but at least remembering and reinstalling all of my extensions won't be an issue.

UPDATE: Not all of my extensions came back -- most of them did, though.