Having recently upgraded to the latest version of FireFox, I noticed all my spell-check facilities had disappeared - so checking my settings I noted that I now had to re-install my dictionary. For anyone else who notices their spell-check facilities have gone, here's how to get them back
On a page where you are allowed to enter text (i.e. like at the bottom of this page in the "quick reply" box), right click anywhere in that box/area and select "Add Dictionaries". You will then be taken to the Mozilla add-on page where you can specify which language pack you want to install.
For me personally, it was English (British) so you simply hit the "Install Dictionary" link and follow the prompts to add your dictionary pack to firefox!!. Once it's installed, you may need to perform the step in the first post to actually enable it again