Slightly annoying it is per window though. I sort of expected the next window I opened to be able to 'boot' up an existing group. This seems to be just tab grouping rather than projects in any meaningful sense. They are only in session data and they don't sync.
Turned out I didn't, and Safari has lots of great features that are well thought through.
I still use Chrome for web dev, but it only runs a single tab with the web app I'm working on.
Try new things! :)
Now, if only they could fix bugs hanging around for more than 6 years, it would be cool too. ie: https://bugzilla.mozilla.org/show_bug.cgi?id=488725
In my opinion, managing tabs as a tree allows greater flexibility and enables one to see the "history" behind tabs (which tab opened which other tab).