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Firefox 3 / Ubuntu 8.04 - Font Size Fix:If you’ve been seeing strange font sizes while using Firefox 3 on Ubuntu 8.04, this about:config setting should fix it - change “layout.css.dpi” from -1 to 72. After this, restart Firefox, and you should see most sites in the correct font size. Found this gem on ubuntuforums.org

Firefox 3 / Ubuntu 8.04 - Font Size Fix:

If you’ve been seeing strange font sizes while using Firefox 3 on Ubuntu 8.04, this about:config setting should fix it - change “layout.css.dpi” from -1 to 72. After this, restart Firefox, and you should see most sites in the correct font size. 

Found this gem on ubuntuforums.org