Personally, while I've been using Tab Session Manager for years, I've had lots of issues with it. There have been a few times where all my sessions would be completely wiped after restarting Firefox or having it crash, which is why I have the backup folder setting enabled. Very occasionally within the past few years, but much more often lately, when I'd try to restore sessions, all of which consist of 100s of tabs, I'd find a bunch of tabs missing, being replaced with blank new tab pages. Also note that this has happened across different computers and Firefox installations.
Looking at the changes / commits made to the extension since its last update made last September, the developer has only updated the readme page and a few other non-code related things. there are hundreds of unaddressed issues on the repo.
It seems like I should really move away from it, what are alternative extensions you would recommend?
People have already explained it but I have gotten the blank page thing with chrome and firefox with their integrated open tabs previously open thing. Mostly it happens with pages that are logged into something. As for tab session I have used it for a long time and I actually had some issues with it years ago especially playing well with power tabs but its been fine for a long time. When I did lose pages long ago I was able to just go to the last save point before the most current which was generally almost the same. Again though I personally have not had issues with it for a long time. Outside of the blank page thing but again I have seen that with all tab recovery things including the native ones.