This would help: http://www.mozilla.org/unix/customizing.html#usercss
It's been a long time since I had to use it, so some of these points may not be correct. You can always experiment.
- You can use the unit point (eg 12pt) for specifying font sizes, you don't have to use mm (millimetres)
- You can remove some sections that you don't need from the file (eg when you are happy with defaults)
- The files are placed in a path like ~/.mozilla/default/8qke875o.slt/chrome. Make them if necessary.