I used userChrome.css code to set the font size for the interface, and I set a minimum font size for the content of the web pages. Those sizes are independent of the operating sytem settings for me.

You can use an extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/

Use this extension to adjust the font size for the user interface:

• Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/theme-font-size-changer/

See also:

• http://kb.mozillazine.org/Zoom_text_of_web_pages