I have been very happy using Tree Style Tab and I heartily recommend it.

However, with the latest version the "Create Group from Tabs" command was removed from the context menu and moved into another companion addon, rather than using the already built in function that lets you customize the context menu.

Further, this new addon is not monitored by Mozilla - which obviously makes the addon a no go for those of us who are concerned with security.

Luckily the old version is still available - here's hoping there's no security risk in using that one!