Max Transmission Units, a setting in your router:check your ISP site for their recommended settings.
Also, hunted around for things I have done to speed up FF in the past.
you might want to try:
Type about:config into the firefox address bar.
Use the filter to search IPv6, and set network.dns.disableIPv6 to true
(bearing in mind that IPv6 is coming sometime soon...maybe..)
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