Seems like IndexedDB was updated in Firefox v74. This addon stores sessions in IndexedDB. And old DB files don't work with new version. That's why we have errors like "IndexedDB error" or instantly closing Tab Session Manager window. Sessions are stored in Firefox Profiles foler. If you backup them and reinstall the addon it will start working fine. To recover them you need to install old version of Firefox somewhere, install Tab Session Manager, add backuped sessions to it (google for instructions by sienori on github) and export it to .sessions (JSON) format. Then you'll be able to import them to freshly installed Tab Session Manager in Firefox v74.

A very handy add-on for your firefox browser

I just added it and it makes my life better! Great way to save multi-window sessions.

very helpful! has a lot of options, like this addon a lot.

I really like this extension. the old session manager I used was good, but this exceeds it. Nice work! Don't know how much work it would be, but the ability to edit session names or tab titles after saving would be a bonus.

Thanks for adding the startup function. For me, that was all the extension lacked. I now consider it perfect. Congratulations!

(7 months ago I had written:
Best session manager, but an important feature is lacking: the ability to start Firefox with a specific saved session.)