Qale (and others) to get Firefox to spellcheck in the Subject line ...
well, any single line field actually.
open a tab and type about:config (then enter) in the address bar
make a promise (you'll see)
type spell in the filter box
find the layout.spellcheckDefault entry
double-click on that line
an input box looking for an integer should pop up
change it from 1 to 2
supposedly, you have to restart FF but it worked right away for me.