Firefox has built in spell-checking for text boxes, but I find that it doesn't recognize the FDR boxes automagically and you need to right click in the box & select "check spelling". Then it will find and underline any/all spelling errors.
If firefox is not automatically checking your spelling, check that you have the appropriate dictionaries installed in the addons.
"We thought we knew everything about everything, and it turned out that there were unknown unknowns." - Richard Fisher, NASA 2009