I think that in Linux the font that Thunderbird uses for menus, message list etc. is the font set for the desktop. I have Xubuntu with the desktop font Droid Sans 10 and that's what I see in the Thunderbird window, except in the message reading pane which I have set to Droid Sans 14. Mind you I don't have a theme changer add-on so I don't know what that does or how it works.

How do I change the Linux font on the desktop? I don't know much about Linux, pretty new using it, I think it is GNU? Gnome? Does that sound familiar?

Different Linux distros come with a whole range of desktops, I'm afraid, which is great for user choice but it makes it harder to provide assistance. Yes, Gnome is one popular desktop. The most common distro is Ubuntu, I think, and that has the Unity desktop. The distro I use has the XFCE desktop. If you do have the Gnome desktop, then you can find instructions here: https://wiki.ubuntu.com/UbuntuGNOME/HowTo (there's a link about how to change the fonts and font sizes)

I found it! There is a Tweak Tool under Utilities where I can change the font. It worked! Yeah!! Thanks for your help.