In FireFox 21.0 I'm seeing some minor issues with the message area width. I typically run FF in a window quite a bit smaller than full-screen size.

I've attached two images that show what's going on. The first one is a message that's "sort of" OK - I can see all of the message text without scrolling left or right. However, it is actually a few pixels too wide - FF puts up a horizontal scroll bar, as you can see at the bottom of the image. If the area was a few pixels narrower the horizontal scroll bar would disappear (freeing up vertical space).

In the second sample image the message area is definitely too wide and text at the right edge can't be viewed without scrolling horizontally.

On a positive note - during testing I encountered a message that had a wide code example in PRE tags, plus some regular text not in those tags. The PRE text was too wide for my browser window, which caused horizontal scroll bars to show, which was as expected. However, I was impressed that the regular text fitted properly in my browser window and was fully readable without horizontal scrolling. This was an improvement over the past, when the regular text would also flow out to the full width of the PRE code, and it required horizontal scrolling to read the regular text. I haven't viewed a message like that since, but I hope that improvement is still in place.