Interesting, don't test on Firefox before. I believe that new version, which not use int10&Long can achieve your speed. I'll write here when update sources.
On my computers, firefox is even faster than chrome. Testing on my Notebook (Core Duo 2.4GHz):
Chrome Version 31.0.1650.63 m
Speed test
Javascript needs 352.6ms/sign
Javascript needs 293.3ms/verify
Finished
Firefox Version 26.0
Speed test
Javascript needs 282.1ms/sign
Javascript needs 228.3ms/verify
Finished
I think the speed could be improved a lot but it will take time.
What are you replacing the int10 and Long with?