[olejorgenb]

I like the the https://josh-berry.github.io/tab-stash/ extension. Not exactly what you want, but IMO a very well-done extension in that space.

Firefox handle lots of tabs very well though since the tabs are lazily loaded. Only issue is that the session system once in a while mess up and you lose everything.

[lionelw]

Just adding this here: I created a multi-window management addon Winger[0] which lets you treat windows as 'tab groups', and it has a stashing feature (windows <-> bookmark folders / tabs <-> bookmarks) similar to Tab Stash. Despite the ostensible scope overlap, I know there's at least one person (not me) who uses both, setting Winger's stash home folder to be the same as Tab Stash's to achieve a manner of integration.

[0] https://addons.mozilla.org/en-US/firefox/addon/winger/

[Cyuonut]

Now you know a second one.