No, I would say that the comments (at least collectively) convey the information that Thunderbird really has a problem, and that the developers are concentratin

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.