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.