This article will show you how to change your menu font size and type in Thunderbird. This can't be done with the GUI so the only way I found to change it is to edit the userChrome.css file. The userChrome.css file sets the display rules for web content and is located in the sub-folder called chrome in your profile folder. As with userChrome.css, this folder does not exist by default, so you need to create it before you can start adding your preferences.
First thing to do is locate your profile folder. This is where Thunderbird saves all your settings on your hard drive.
On Windows Vista/XP/2000, the path is usually %AppData%\Thunderbird\Profiles\xxxxxxxx.default\, where xxxxxxxx is a random string of 8 characters. Just browse to C:\Documents and Settings\[User Name]\Application Data\Thunderbird\Profiles\ on Windows XP/2000. On Windows Vista C:\users\[User Name]\AppData\Roaming\Thunderbird\Profiles\.
On Windows 95/98/Me, the path is usually C:\WINDOWS\Application Data\Thunderbird\Profiles\xxxxxxxx.default\.
On Linux, the path is usually ~/.thunderbird/xxxxxxxx.default/ where xxxxxxxx is a random string of 8 characters.
On Mac OS X, the path is usually ~/Library/Thunderbird/Profiles/xxxxxxxx.default/.
For example in linux:
mkdir -p ~/.thunderbird/xxxxxxxx.default/chrome
where xxxxxxxx is a random string of 8 characters.
In your newly created directory create a new file called userChrome.css with your favorite text editor.
Put in the font-size line below if you want to change the size of the font or the font-family line below if you want to change the font type. Then save it and exit.
* {font-size: 12pt !important;}
* {font-family: Arial !important;}
Restart Thunderbird or Mozilla and it will have the new settings.
