You can change the fonts in the Firefox Preferences and you can improve rending
from the Ubuntu settings. Sometimes Firefox can be made unstable by adding
certain extensions. In my experience I have never had a problem with Firefox
that has not been tweaked or added to. Instability seems to develop over time,
as you change it.

It may also have something to do with other things that you have running as
well. I was having a problem on the eeePC with Compiz running and Forefox
freezing. I shut off Compiz and the problems with Firefox went away and
everything else seemed to work better as well.