Well I have both my monitors set at 1600 x 1200, and the text is teeny. I can and do use the side wheel on my mouse for zooming, which enlarges the text, but not in the reply field. I can position the cursor outside the reply field, and then ALL text, inside and outside the reply field is zoomed, but not proportionally.
So I end up with huge text outside, just so I can have normal text inside. It's a real pain in the ass, but I have to go through that in order to see if I'm misspelling words or not.
(no, I do not want to increase the font size in either my browser or Windows, for all kinds of reasons)
On other sites it's not a problem, since those words are highlighted and easy to see, but, here... you know.

Before I typed up this thread I had just posted elsewhere on this site, and had to edit it three times for spelling errors. It pissed me off, and it wouldn't have happened if it weren't for this lack of support for this feature in Firefox.

Two things I doubt will happen:
1- Nobody from the staff is going to explain just why they have no interest in changing it.
2- There will not be a built-in spellchecker created for this site.

Reading just about any random page in the Marketplace will demonstrate that proper spelling isn't a priority here, anyway. Or grammar, or even sentence structure...