Some Firefox extension(s) may also mess it up: for example, on my browser (a Firefox derivative), all posts on this forum have their first line cut in the middle with the end of it "sent" on the second line. But this happens only when using the main profile, and not with profiles with less extensions nor on other web browsers. I'll try to understand what occurs one day, the day I'll feel really uncomfortable with it

Maybe you should try another profile (en, fr) to see what happens in a "naked" browser?