I suggest trying this add-on: [https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/ Theme Font & Size Changer :: Add-ons for Firefox] If that doesn't give you enough fine control, there are more complicated solutions, as you can see in this thread: [https://support.mozilla.com/en-US/questions/831630 How do I change browser text size? Not content but actual browser. | Firefox Support Forum | Firefox Help].

For your information, in case you are interested. Firefox 4 doesn't run on a PowerPC Mac. Firefox 4 requires at least OS X 10.5 and an Intel Mac. * http://www.mozilla.com/firefox/4.0/system-requirements/ For an unofficial Firefox 4 compatible version (TenFourFox) that runs on PowerPC Macs with OS X 10.4.11 or OS X 10.5.8 you can look at: * http://www.floodgap.com/software/tenfourfox/ ---- In addition to the above posted extension for the user interface (toolbars), you can use one of these extensions to set a default font size and page zoom on web pages: * Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/ * NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/