I had the same issue one day all of my tabs where gone, but by opening the browser with just the 1 tab showing a search page, that synced to my other devices and my 30+ tabs went poof. If you have another device with Firefox you can look at the View Sync Tab to see what machine names are there and what tabs.

Click on the icon for the Firefox Account Click on Synced Tabs Click View Sync Tabs Sidebar to see what other devices you may have.

Ok. I forgot to mention 1 thing. I have another Mac (2) and i also upgraded to Big Sur, and Firefox works perfectly on that machine. Going back to the original Mac (1) using google chrome to type to you, I go into the firefox account > synced tabs and nothing populates. I had even hit Sync Now and nothing populated. I try to go to Firefox account > Connect another device and then accounts.firefox.com wont load. I tried to go to youtube.com to test and the page pulled up a proxy error loading. I check the proxy settings on both macs and they are identical. Advanced wifi settings under Network are the same on both machines. I just now cleared history, cache, etc and that didnt work. I deleted firefox and re-installed. when I opened it, clicked on youtube bookmark and i got the unable to find proxy server message.

The tabs are gone. Game Over.

I dont follow. Firefox will never work on this machine again?