OK, I found spell checking not only for PFF but for any typing you do within Firefox as well and seems to work fine. It was recommended in the PFF handbook if the user didn’t mind using a beta product. It includes in-line as well as regular spell checking.
The easiest way to get the latest version is to go to the PFF handbook as described in the previous post and download it from there. Remember to read the instructions carefully since the english dictionary has to be selected before anything works and getting to the menu that offers suggestions for misspelled words (in-line spell checker) is ctrl + left-click rather than right clicking which I assume as to not to interfere with Firefox’s regular right click context menu. This seems to work ok once you get used to it.