Update: See Sage-Too is dead, long live Sage++ for links to a new version of Sage that works with Firefox 3.5!
So, I upgraded to Firefox 3.5 RC3 today. I was disappointed (though not at all surprised) to see that my second-favourite add-on (after the essential Oldbar) had stopped working. The Sage-Too Firefox RSS Reader extension is packaged to run with 3.0b5 to 3.0.*, it seems.
The good news is that manually repackaging the plugin and reinstalling it with 3.5 does work fine. As far as I can see the project is orphaned – a real shame in my opinion – so if anyone’s interested in a copy, leave a comment here and I’ll whack a page up with instructions (and perhaps even a copy if I can make sure the authors won’t mind.)