It seems that Firefox 3.x interprets font sizes a little differently than Firefox 2.x. I found that where I had specified ‘pt’ sizes in stylesheets and it looked fine in Firefox 2.x, they get blown out of proportion and huge in Firefox 3.x. The solution is to use em or ex measurements for fonts.
The symptoms didn’t appear to be caused by a font issue at all – tabs getting shifted out of whack in a web application chrome or text getting cut off in weird places. I was really glad to have figured out why it was happening and I’m also super-happy the fix is so easy. :)