Hi Raptor,
I am a major fan of firefox, better than sliced bread as far as I'm concerned and I had a quick read of your problem. I followed what you desired to do and it worked no problem. The only thing I can think of is....did you select 'dictionary.com' or the word 'Dictionary' cos it worked fine for me when I chose the latter. It also worked with Alta Vista. Can't see it being a problem with FireFox, it's TOO good! Persevere, you'll get there.