New alternative? Rated 5 out of 5 stars
There's a new extension for FF 4.x that seems to headed in the direction of TSB's former ability to display tab previews in the sidebar. It's called Vertical Tabs. Right now, tab previews are only "planned", but if enough requests for this feature also pointed to TSB's open-source code, perhaps VT's developer could accelerate the inclusion of this feature. You can reach VT's developer at firstname.lastname@example.org. In the meantime, I'd encourage you to check out Vertical Tabs and follow its development.This user has a previous review of this add-on.