It is possible to change the display font and the size of the font on the Thunderbird User Interface. However, it is not something that can be done within Thunderbird. You will need to modify your userChrome.css file within your Thunderbird  profile folder. Follow these directions below to make the modifications:

1. From within Thunderbird go to Help > Troubleshooting Information
2. In the Application Basics section next to Profile Folder, click the Show Folder button
3. An Explorer/Kernel Window should open to the location of your profile folder. CLOSE Thunderbird
4. Go the Chrome folder and locate the file userChrome.css and edit. Note: Some installs of Thunderbird may not include a chrome folder. If this the case create a chrome folder, download a fresh userChrome.css file here and place it in your chrome folder.
5. Add the following lines of code to the bottom of your userChrome.css file:

/* Global UI font */
* { font-size: 10pt !important;
font-family: Verdana !important;
}

Note: You can make the font-size bigger or smaller depending on your resolution. You can also use any font-family (Times, Arial, Geneva, Verdana).