Compatibility broken with nightly 21? Rated 5 out of 5 stars

With Firefox Nightly 21.0a1 (2013-02-03) I don't see the group tabs, only a line with a "+", "hibernate group" and "-". Is the compatibility broken?If so, may someone suggest an analogous extension (groups, hibernated groups, tabs not getting loaded upon session restore (the most important feature for me)).In Firefox 18 the group tabs are there, but the whole browser works way slower, almost unusable.

One Ken Thomas, could you look into this?

This user has a previous review of this add-on.