Since I believe ffox 16 (on both OS X and Win7), I've found that whenever I correct a misspelled word in a Gmail compose window using rich text — by backspacing and retyping — ffox will highlight *the retyped part of the word* as being misspelled. This has been driving me nuts. I've since tracked down the problem to the rich text environment within ffox itself, I think. Examining the original source of the email message in Gmail, you can clearly see the 'corrected' text in the MIME HTML encoded message, and using view selection source prior to sending. Firefox is inserting <font> tags at the point where I begin retyping! So my guess is the spell checker fails for the 'new' word, where it is not valid in its own right.

To illustrate, I wrote a simple new email with the text 'what is this nonsense', where 'this' and 'nonsense' were first misspelled (missing letter), then corrected by backspace and retyping. </font>