A long time ago (and I do mean a *long* time ago) I posted that I was having problems with this combination. I've been chatting on and off with the ASP.NET team about this as we tried to track down the issue on a simple repro I'd set up in the office. Progress has been very slow for a variety of reasons...
Then today, entirely by chance I came across this post in Dan Wahlin's blog which describes the exact problem, the cause and a workaround by changing a FireFox setting while doing localhost testing. I tested this on my repro and sure enough, FireFox starts rendering pages at "normal" speed (ie not taking an age as it does without the workaround in place).
Apparently it relates to an IPv4 address being returned by a DNS server when an IPv6 address is requested. The net result is FireFox recovers from this but it introduces a significant delay. The workaround is to disable IPv6 support in FireFox while doing localhost testing. Full details are in Dan's post.
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Actually a better solution is to use the Network.dns.ipv4OnlyDomains setting. This leaves IpV6 on and only enforeces ipV4 for the domains in this comma separated list.
I actually blogged about this a longer time ago than this solution as well.
Thanks Ben - sorry I didn't spot yours. Dan actually blogged about it in June it was just that I didn't come across it until much later! Mike