Undocumented requirement Rated 4 out of 5 stars
There is an undocumented requirement that I stumbled across before this worked properly for me: you must set View | Sort By... | Date.
I initially had TB set to sort by a different method (Order Received), and this extension did absolutely nothing in that case.
If I had to guess I'd say you weren't sorting properly. Make sure you are sorted like the screenshot shows: https://addons.mozilla.org/en-US/firefox/images/p/24649/1218576692
Edit: The requirement is intentional and documented. From the support page, http://www.micropipes.com/code/threadbubble/ :
The extension checks to make sure you are sorting by date before it does anything (to keep from messing up your view if you temporarily sorted by another column). If nothing happens after you install, that's the first thing to check.