As someone who used to have hundreds if not thousands of Tabs, Firefox is the only browser that is stable enough and has
the best Tab Session restore. So my guess is that something went wrong if you have to constantly restart Firefox.
Firefox has a sessionrestore.js and also a sessionrestore.bak. ( And also sesstionrestore.bak2 if I remember correctly )[1] It is the only browser that took session restore seriously because around ~16 years ago an idiot constantly went on Mozillazine and Bugzilla to file crash report of lost sessions that could have thousands of tabs.
If your sesstionrestore crashed or get wiped, i.e you restart your browser and those tabs were not there, you could also copy and past the .bak files and rename it as .js. It wont be everything you had since the bak file may have been a few hours old. But at least you got something back.
As to crashing. In the old days it was some tabs in the background doing JS with memory leakage. But if I remember correctly Firefox now pause Tabs that is unused for long period of time. If it isn't background Tabs that is causing problem. I would suggest similar to what I wrote to Animats, make a new Firefox profile and migrate all your History, Cookies etc to the new profile and see if it improve things. This used to be a manual process but I believe recent Firefox has made this easier with Firefox Retune or Refresh ( Sorry all of this are on top of my head you will have to do some Google search for the actual name)
If I were to rate Browser stability, it would be Firefox > Chrome > Safari. On both MacOS and Windows. The same ranking for session restore function where I have lost multiple sessions on Safari over the years. And in case anyone wondering why Safari is not stable and that may not be their experience, it is that something on MacOS ( the OS itself ) sometimes triggers Safari to reload all the Tabs even if they were originally sitting idle. This create huge I/O spike and generally leads to crashing or halt for a long period of time. Other than that it is pretty much on par with others.
[1] Those file and extensions names were something on top of my head so you have to google it to double check