Your issue is that retina uses a pixel doubling technology to make everything look of a size you can actually see, your other monitor as VGA does not support the pixel doubling. What you need is two different video drivers on your device, but with a mac I have no idea how that is done.

Alternatively get a second monitor that also supports retina.

Thanks, Matt, but it did not seem to happen for several years on the same setup (Retina MacBook and older monitor); it seems to have started only after I added the Theme Font & Size Changer add-on and persists even after removing it. It also does not occur with any other program (e.g. Firefox, Apple's Mail, Word...). This make me think profile settings have been compromised.....

Everything that the addon does could be achieved by you writing a css file. So you could get these changes without using the addon.

I don't know precisely how this addon goes about its work, but my view is that if changes can be applied without the addon, any changes it does apply could well persist after the addon itself has been removed.

IOW, removing the addon won't necessarily undo the changes it applied. Your best bet might be to re install the addon and see if you can find an adjustment that works.

But....

If font sizes are declared in pixels, as is done in Firefox and Thunderbird, then if you have two displays where the pixels are of different size/spacing, it's hard to see how it could print the text at the same size on both displays.

I currently have a happy set up where the external monitor has the same logical size (in pixels) as the laptop's built in screen, so I can move apps between the two knowing if the window fits one monitor it will fit the other one too, so no need to be continually resizing stuff. But the laptop screen is physically smaller than the external screen, so necessarily presents everything smaller. I'd love the text on the laptop to be bigger, but adjusting to make it do so would take me to a point where text would no longer fit into the controls. Using a lower resolution on the laptop display would break the compatibility between the two screens. As always, it's a compromise.

Thanks Zenos. I imagine that you are right and that the add-on wrote the equivalent of a css file and it persists. Odd thing is that it changed the default button font color, as well as mucking up font sizing when dragged to the external (and very old) display. I don't mind resizing windows, it is the grossly enlarged text I can't work with.

So, I am looking at uninstalling and reinstalling Thunderbird and only trying to preserve archived messages. Searching the Support Forum now for advice on that. Once I get it reinstalled, I will post back if it solved the issue (and then post to the Add-on site reviews)

As promised..... Only solution was to create a new profile and re-setup the email accounts. Now Thunderbird window does not oddly resize fonts when moved to external monitor (actually, it does resize, but rapidly changes back to "normal" view. It makes me suspect that Theme Font and Size Changer wrote some script that prevented the window from automatically readjusting. I would caution against using the extension...). Also used the ImportExportTool extension to export archives to external drive (for safekeeping) and reimport to new profile (for searching).