Here is an add-on that may help you:

https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/

Email isn't like other kinds of documents such as doc or pdf where you specify fonts that are embedded in the file, and the recipient must use a compatible program to view the document. With email, you embed a generic set of type properties for outgoing mail, and set your email program to display messages that you receive or compose according to your preferences.

For the former, keep the defaults under Tools/Options/Composition/General/HTML, while for the latter, set your preferences under Tools/Options/Display/Formatting/Fonts & Colors/Advanced, for each type of encoding shown under Fonts for:.

http://kb.mozillazine.org/Font_settings_in_Thunderbird

http://www.ramsden.org.uk/9_Type_sizes.html

The add-on mentioned above will let you adjust the fonts for menus, folder names and message lists.

Hi goldensword. Unless I am missing something this has not helped me to find and allocate new fonts to the font collection currently available in Thunderbird. I need to be able locate, install and use 'Sans Serif'.

Thanks