funny thing, that's exactly the problem I was having..ever since I upgraded my Firefox to V 3.0.5, I've noticed a change in the way fonts are handled.
In the past, the default fonts seemed larger...now when I increase them, the whole page expands horizontally and it adds a lot of empty space between text...not really happy with it.
I've played with what I believe are the applicable options in the Settings area but still haven't found a combo that works well for text in actual websites while also working for text in Google search, for example.
And to quote a phrase applicable to this site "I still haven't found what I'm looking for"