While it's come a long way since its inception (I switched to this after a feature-rich and reliable session manager that I used was discontinued after Firefox's engine overhaul), it still seems to act up sometimes with too many tabs open.

Currently, this add-on just duplicates the saved windows I had open and doubles up on my memory usage. It doesn't seem like this is needed anymore with Firefox's native support.

WARNING!!!!! If you have more than 10 tabs open, this will overload the extension. As a self-defence mechanism, the extension deletes the saved session.
It cannot import a saved session containing more than 10 tabs.

In the case of tab groups, this add-on does not preserve original tabs grouping while importing a session: it imports all tabs uniformly, i.e. without any tab grouping. Dear developers, please, correct this fault. P.S. Yet pinned non-grouped tabs are imported correctly (as pinned).

Worked great for a long time, but in the last month or so, the "autosave sessions" feature hasn't worked at all. I've checked my sessions to try to find windows/tabs I wanted to go back to, and the last thing saved was over a month ago even though I know I had Firefox at least set to restore previous windows. But this extension didn't save ANYTHING over the last month+ even though I had it set to do so, so that feature doesn't do anything anymore apparently. I've lost dozens of sessions due to either a bug or recent updates, but either way this is bullshit.

I can't figure out if sessions are tied to virtual desktops when they are saved and is it really important to me

Garbage. Stopped working months ago. Literally never saves tabs. Don't waste your time. It also randomly deletes your sessions. Don't be tricked by the good reviews btw. They're old reviews from years ago when it actually worked.

Misiing the main feature I was looking for - namely support for FF's tab groups. It seems the Chrome version has it. Please add functionality for FF.

Even when the firefox history allows to open closed windows and tabs, there are occasions that can brick this feature.
This addon saved my life already once - and its a good reminder to clear my tabs everytime save enother session with 900 tabs :O

does not work with Firefox 'tab groups'