I had a problem subscribing to my Favorite Addons RSS feed, because my username contains an underscore _ . My announcement feed worked, and so I was able to spot the problem. In a URL, the underscore character should be replaced by _5F00_ which was happening automatically for the announcement feed link, but not for the favourites link. I was able to manually fix it, but other people might come across this bug too.

I'm using FF 3.0.8 on Windows XP.

Thanks for that info, we'll make sure that's fixed asap

This was resolved today.