Firefox’s spell checker has saved my bacon a number of times. Before I had to search around for special tools and services to slap me on the wrist each time I misspelled mississipi..misissippi..Mississippi!
For some reason though, by default Firefox does not do spell checking in all text areas. You can fix that for youself like I did with a little about:config tweaking.
In your about:config (type about:config in the address bar to access) search for this string:
layout.spellcheckDefault
Change the value from 1 to 2 to get spell checking for all text fields and not just the big text area boxes.
