I can't believe that we are days before 2021, and Firefox can still kill your tabs in a split second without anything meaningful in the UI for recovering them.
The scenario is pretty simple : just open a tab in a second Firefox instance, then quit your main Firefox instance (intentionally or not). There you have it : the tabs from your main Firefox instance are lost.

The fix for this is to find the local folder where Firefox stores compressed urls (json lz4). Then, given that you can do nothing with it meaningful, pass it to a LZ4 decompressor so it shows the list of urls. Finally create a new tab for each such url. When this process completes, the tabs are back, except the fact they were not recovered, but recreated by hand one by one.

This begs the question : are we really in 2021 ? or back in Mosaic days ? This is ridiculous state of the art. Perhaps Firefox needs to go down the toilet.