I saw some problems (not error, but notice or warning) about Firefox's theme engine when start on 11.10 .
But I don't think that this makes it slow down too much.
Chromium is faster than Firefox (I am a fun of Firefox by the way). If you are talking about this, it seems to be normal. Try to remove unused add-ons, that may help you make it a little faster (depending to the number of add-ons)
And just my opinion; may gnome theme for ff can cause it too.
Have you seen Firefox Slow Performance ? There is an answered question with a similar question???