Lost online/saved bookmarks after PC Reset, how do restore?

I have reset/reinstalled Windows on this PC many times over roughly the past decade. Every previous time, I would reinstall Firefox, sign back into my Mozilla/Firefox account, enable Sync, and my data would come back: years of bookmarks, saved passwords, payment information, and other Firefox data. I reset my PC again recently and did the exact same thing, except this time everything is missing. Firefox Sync itself appears to be working normally. I am signed in, Sync is enabled, the appropriate categories are selected, and I can see that Firefox is syncing. The problem is that there seems to be nothing from my old Firefox data to sync back down. I am also fairly confident that this is the same Mozilla account I have always used. This is the only email address I tested that behaved like an existing account with an existing password. Other email addresses I tried appeared to be treated as new accounts and asked me to create a password. So I am not really looking for basic instructions on enabling Sync, manually syncing, checking the last sync time, or selecting which data types are synced. I have already gone through that. What I am trying to understand is how an account that has successfully restored this same Firefox data after numerous previous Windows reinstalls can suddenly have none of that historical data available. Specifically: • Can previously synced Firefox data disappear from Mozilla's servers even though I did not intentionally delete it or disable Sync? • Could reinstalling Firefox and signing into a newly reset/empty Firefox installation somehow cause the empty local profile to replace or wipe the previously synced server data? • Are there circumstances where Firefox Sync effectively starts over with an empty dataset while continuing to use the same Mozilla account? • Is there any server-side recovery, previous Sync state, historical copy, encrypted backup, or other mechanism Mozilla support can access when the current Sync dataset appears to be empty? • Is there anything left to investigate on the old Windows installation/reset itself, such as a previous Firefox profile, Windows.old, recovery data, or another local copy, even if Firefox Sync no longer has the information? • Is there any way to conclusively determine whether this account previously contained the missing bookmarks/passwords, rather than simply confirming that I am currently signed into the account? The important distinction is that Sync is not currently “broken.” It appears to be syncing normally. The problem is that years of data that Firefox Sync had repeatedly restored for me in the past now appear to be completely absent. I am trying to determine whether the old dataset may still exist somewhere, whether something could have caused the Sync dataset itself to be reset or replaced, or whether there is any remaining recovery path for the previous Firefox profile/data.