Lemme try that! (It actually worked without a browser restart! I just had to reload the page once.)

Okay, some may indeed be that there's a comparison, but the subject line's font size is not scaling with magnification, while the Courier (body-text) is. And it's definitely a problem when I'm having a bad eye day.

I have changed Firefox preferences to... There we go. Minimum size 14, and do NOT allow pages to override my preferences. (This page, I think, was overriding my preferences.) Subject line's font size still does not scale when I magnify via command-plus, nor does it shrink likewise. (Got down to 67% and I still have what I assume is Helvetica 14 point in the subject line, even though the Courier has gone down to something smaller.)

I dunno if you want to pursue the subject-line text not obeying Firefox magnify/shrink, since my problem is solved, or just file it as Known Weirdness. *grin*

Thanks for reminding me that Firefox has font-size preferences that can be independent of system preferences! -_-

...I'll restart the browser, just in case, and edit this if it behaves any differently.

Edit: no changes; Subject line's font size laughs at magnify/shrink commands, even as the rest of the page behaves reasonably. *shrug emoji*