Quick Fix
Just change
http://localhost:XXXX/YourSite/Here
To
http://127.0.0.1:XXXX/YourSite/Here
Much better IMO than disabling IPv6 settings in Firefox, what if you want to use IPv6?
More Permanent Solution
If you want a slightly more permanant solution, you can edit your hosts file, normally C:Windowssystem32driversetchosts
on Windows, and add a rule for localhost:
127.0.0.1 localhost
Customising Firefox Specifically
If you do decide you want to disable IPv6 in Firefox itself, there are details of that here:
http://kb.mozillazine.org/Network.dns.disableIPv6
Edit: Actually, if you want to fix this in Firefox, rather than disabling IPv6 entirely as in the link I just posted, or as specified by the other answer to this question, give this a read:
http://theycallmemrjames.blogspot.com/2010/09/firefox-is-really-slow-testing-sites-on.html
This shows how to set a rule for localhost to only use IPv4, so your browser may still use IPv6 as it needs.
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