Must-have, DOES work w/ FF4 Rated 5 out of 5 stars

As far as I can tell BarTab does work with FF4 once the "max version" details in the XPI are modified. I have it installed and it appears to work as expected. So here's what you do, beginner-style:
(1) download the XPI from here or SnapFiles
(2) open about:config and navigate to "browser.sessionstore.max_concurrent_tabs", right-click and select RESET
(3) use a zipfile manager (these instructions are for 7-Zip) to open the XPI, right-click "INSTALL.RDF" and select EDIT
(4) locate the "maxVersion" tags and change the numerical value to " 4.0.* " w/o quotes
(5) save the updated archive (yes to update prompts) and install to FF
(6) return to about:config, filter "browser.sessionstore.max_concurrent_tabs" and change to value 0
(7) configure BarTab to not load tabs opened in background
Voila!

One suggestion Rated 4 out of 5 stars

It would be great to be able to set BarTab options on a per-site basis -- so it could be in place and enabled but nonfunctional on given sites -- eg, "do not affect the operation of child tabs from parent site www.x.com".

This review is for a previous version of the add-on (1.5.1). 

Maybe

Thanks for feature suggestion, though I'd really prefer it if you guys got in touch with me directly (e.g. email) so I can follow up.

I'm not sure I understand exactly what you mean. Certainly more flexibility would be always great, but I'm worried about making the UI (especially the preference settings) too complicated.