and after a reboot does it still do such a thing?

I reboot at least once a day and it has not resolved the issue.

There were reports of this years ago, but that bug was fixed.

Thunderbird usually does not have much in the way of font names to navigate through. as HTML does not spend much time on Fonts. After all fonts are local. Very little point specifying them. If the recipient does not have that font installed they see something rather nasty usually.

So does your message contain text that originates somewhere else, like a word processor where fonts are very relevant? or a signature that started like in Microsoft Word?