There's another thread about the RC (which is newer than beta5 I believe), and that mentions a few about:config tweaks.
I tried the second, it seems to help, but it's hard to tell since Firefox 4 performance is a bit hit and miss in my experience.

These are the relevant posts:

http://talk.maemo.org/showpost.php?p...2&postcount=23
http://talk.maemo.org/showpost.php?p...7&postcount=38