This is a great extension and I use it constantly, both for tab grouping and tab backups (which come useful with Firefox rarely being 100% stable for me). I have some comments about backup mechanism though:
- (quite major) I sometimes end up with a bunch of tabs not assigned to any group - and these don't get backed up! I have known about this for a while, but still it occasionally trips me up. It would be much better if the backup included "ungrouped" tabs as well, maybe in a new dummy group. This behaviour can be made optional if for whatever reason it's expected that someone might want to opt out.
- (minor) Backups don't save tab distribution between windows, so after crash I have to manually recreate windows and shuffle the tabs between them, but that's more of an inconvenience, as long as all tabs are restored.

And again, thank you very much!