If you keep up with the Mozilla nightly builds, you may find that the spellchecker has gone a bit wonky. Since approximately from the 22nd, the spellchecker no longer recognizes capitalized versions of words already in its dictionary. For instance, if you start a sentence with “The”, “This” or “So”, it'll think you've misspelled the word.
You can still add the word to its dictionary and it won't bother you about that one again, but this is enough of an annoyance to me that I don’t think I’ll be downloading the nightlies for the next couples days until this one is fixed. And, it looks like it should be fixed soon — there’s already a patch awaiting approval :).