Firefox Problems - webmail crawls

Looked on the forum and I've seen several people reporting problems but no 
sign of a solution.

I'm at work and can get to webmail.spamcop.net to check my mail at what I'd 
consider normal speeds if I use IE6.  If I use Firefox, it gets there, but 
it absolutely crawls.

Does anyone have any suggestions?

We are using a firewall/proxy and whilst I guess it could be to do with 
that, I don't see how it would be selective about which browsers it works 
with as every other site I use is fine with Firefox (barring ones using IE 
specific content of course).

cheers,
Paul