Is your data on another device? My data is on several devices and versions of Firefox. see old screenshot

FYI... SYNC IS NOT A DATA BACKUP SERVICE Sync info... https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer https://support.mozilla.org/en-US/kb/how-do-i-choose-what-information-sync-firefox https://www.mozilla.org/en-US/firefox/features/sync/ https://hacks.mozilla.org/2018/11/firefox-sync-privacy/ Sync is not a backup tool but it is a good sync/transfer/move tool https://support.mozilla.org/en-US/forums/contributors/707681

Yes, I have several devices that I use Sync with. As far as I can tell, they've always worked fine.

What OS is it stuck syncing on, did you try this option? see screenshot

I'm using the latest version of Linux Mint (22.1). Yes, I tried disconnecting and reconnecting several times. I also played with what is being synced. I even went so far as to uncheck everything. The result is the same: stuck Syncing. Quitting Firefox always results in a short delay followed by a crash report. Crash report details is a bit long, but include: "MozCrashReason: Shutdown hanging at step XPCOMShutdownThreads. Something is blocking the main-thread.".

Is there any log data I can find that would illuminate the root cause. The weave logs folder is empty.

Try downloading Firefox from Mozilla and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/desktop-release/ Just download, unzip, and run firefox-bin from the folder. You can also try your OS from a live usb stick. Sync and see what happens.