I previously gave this 1-star because it would get stuck loading, but it turned out to be a Firefox issue - not an issue with the extension! 5-stars now, best Tab Groups extension for FF!

If your monitor refresh rate is high (and on new MBPs that's true) you need to adjust the layout.frame_rate config variable in about:config to equal your monitor's refresh rate. Crazy, huh? For new MBP's, that's 120.

This is the best Tab Groups extension hands down. I tried them all. This is it.