It Works! Rated 5 out of 5 stars

After an almost immediate, polite and informative response from the developer, I did the following: disabled Theme Font and Size Changer, installed Bigger Toolbar Buttons, works like a charm, re-installed Theme Font and Size Changer, everybody's happy now. Haven't tried it on the Linux Mint side of the computer, but I expect no problems.

Now a request: there are still a bunch of tiny icons to click on, such as the x on the tabs, which do not change size. Either its my set up, or Thunderbird's folks haven't looked at the needs of large high res displays. It could very well be that I need to change something, any thoughts appreciated.

This user has a previous review of this add-on.

Large Theme

Well, what's really needed is a large theme for Thunderbird. It's something that I've always wanted to make but have never had the time to do.
I have poor eyesight plus my screen resolution is 1920x1200 so I've had to customize a lot of things for myself including using my own large toolbar buttons, close buttons, icons and other things.

If you use Stylish, the following may be helpful. It isn't a perfect solution, but it may help and you could use your own image for the close button and make it whatever size that you'd like, or just add a background color or border for better visibility of the close button.

.tab-close-button > .toolbarbutton-icon{
min-width:30px!important;
min-height: 30px!important;

}