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.
Will it rise from the dead? Rated 5 out of 5 stars
Outstanding addon!In case you've missed it, the developer stated his intention to not do any further development on TSB nearly two years ago. I wrote him to find out if he's sticking to that decision (which would mean no FF4 version)--unfortunately, he is.
As TSB is open-source, however, the developer stated that anyone who wants to work with the source code on a FF4 update is welcome to do so. The source code is available through the developer's web site (see above). I'd recommend that any and all TSB fans ask your favorite FF addon developer(s) to consider taking on this project! I've contacted the author of Tab Mix Plus encourage him to do this very thing; if you'd like to add your voice to that request, you can email the TMP author at email@example.com.