Are there any addons/extensions for Firefox that do?
Okay, technically, repeated words are not "bad spelling", so I massaged the title of this question a bit. Hopefully, that will appease you semantic types a little :)
Out of the box, Firefox does not do grammar checking. Only incorrectly typed words.
Don't think there is an add-on that does that though.
There is now a grammer checker add-on available for firefox. I've not used but I plan to: