Great addon, has been updated to issue features/fixes addressed to be lacking in the previous reviews.
Seemingly the most modern and useful session manager.

very important productive extension. Thanks for save my tabs.

Its a life saver and with cloud sink, its also has life backups.

Now that it supports cloud sync (the only thing it was missing), it's surely the best tab session manager out there, and I've tried many!
Thank you for your last update, hope you're going to support other cloud storage too as Dropbox and, why not, something open like Nextcloud.
Great work!

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.