Ok, I think I found the post I was remembering, and it seemed to me like understandable frustration at bad software decisions:

https://support.mozilla.org/en-US/questions/1269361#answer-1...

Note that the css hack discussed in the thread helps by controlling some of the sizes, but not enough of them. You end up with a mishmash on the screen.

Hmm, this one further in the thread is more of a rant:

https://support.mozilla.org/en-US/questions/1269361?page=2#a...

There are a few other responses to the original comment, and one of the mods deleted some naughty words from the rant, leading to this comment:

https://support.mozilla.org/en-US/questions/1269361?page=2#a...

This one puts the blame where it belongs:

https://support.mozilla.org/en-US/questions/1269361?page=3#a...

Finally, this sums up the wider situation:

https://support.mozilla.org/en-US/questions/1269361?page=3#a... : "Unfortunately, this is typical with Mozilla: every "upgrade" is a downgrade in some important aspect of the program."

That rings true for Firefox as well.

Ok, I'll stop pasting links from that thread here but I found it to be worth scrolling through, to see the user frustration that Mozilla derps around so much with advanced features without managing to get something as basic as font sizes working properly. I can use Thunderbird with just some annoyance here and there, but for my mom (her vision isn't so good any more), Thunderbird is unusable and there seems to be no sane way to fix it. (One of the suggestions was to fork the source code: thanks a lot).

We're agreeing, possibly. I am saying those comments are only outlets of emotion, using a common form (Internet rants), telling us about the author (who is frustrated, etc.) and very little about the problem. You say, "it seemed to me like understandable frustration".

No, I would say that the comments (at least collectively) convey the information that Thunderbird really has a problem, and that the developers are concentrating on the wrong things. It is also really true that new versions introduced regressions by breaking the workarounds for font issues in older versions, and that the devs don't seem to care.

The purpose of a mail reader is to read mail. If the fonts are too small to read and can't be enlarged without a lot of hassle, then the mail reader has a serious usability issue no matter what other features it has.