Firefox does have a spell checker inbuilt in to the latest versions.
Tools - options - advanced
Make sure 'check my spelling as I type' is ticked.
Now, start to type a reply in the AnswerBank 'post an answer' box and right click.
You should see 'check spelling' is ticked,and underneath that you should see 'Languages'. Make sure it is UK English.
If it isn't, right click again and click ;languages' and 'add dictionary'.
You may have to restart firefox.
I have no idea why IE isn't working for you.