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Jan 4, 2012

Set the Menu/Application/Interface Font Size for Firefox or Thunderbird on Mac OS X

Firefox LogoSome of my Macs have a very high screen resolution. Therefore the font size of the menus for Firefox and Thunderbird seems a little small. It turns out you can actually adjust the size of the application fonts in Firefox or Thunderbird using CSS.

To change the default fonts, edit the userChrome-example.css file. The trick is to find this freaking file. For Thunderbird, the file does not exist and you must create it from scratch. Things are easier for Firefox.

First, here is the CSS code you need to add to userChrome-example.css to change the default font.
* { font-size: 11pt !important; }

This sets the default font size for the interface to 11pt. Next you will need to create the file in the appropriate directory for each application.

Thunderbird
For Thunderbird, first find the Profiles directory.
/Users/YourUserName/Library/Thunderbird/Profiles/someRandomeString

In this directory:
  1. Create a chrome directory.
  2. Create userChrome-example.css in the chrome directory include the CSS code shown above

Firefox
For Firefox, the chrome directory already exists and actually includes some sample CSS files.

Your profile can be found in:
/Users/YourUserName/Library/Application Support/Firefox/Profiles/someRandomeString

Change into the chrome directory and create the userChrome-example.css file as described above.

That is it. With this code, the default font size for menu and other application text should now be set to 11 pts.

For details on modifying specific parts of the interface, check out this page: http://kb.mozillazine.org/Pane_and_menu_fonts
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