I just tried to download Firefox from their website, and unpack the tar. Wola! The spell check worked. It seems to have something to do with permissions, prob the use of "file://" when trying to open the files? Not sure how to fix this issue though.
I just tested to link ~/Download/firefox/dictionaries to /usr/share/hunspell, and that actually worked. But not if I do the same in the installation folder (/usr/lib/firefox).
Another odd thing is that i can point my browser to "file:///usr/share/hunspell/en_GB-large.dic" and grab the file, so it should work!?