I’ve tried the firefox 3 beta a couple times on my linux box. Every time I try it the fonts and toolbars are obnoxiously big. I couldn’t find any help on this. It seemed like a dpi issue, so I went to about:config and searched for dpi. There it was layout.css.dpi. Mine was set to -1, I changed it to 150 which seemed about right.
Type about:config in you browser window. Then type dpi in the filter text box and double click the layout.css.dpi to change the value. I changed mine to 150.