|You didn't say wheter this was:|
a new problem
a new installation of FF
a new ISP
behind a firewall
Try to ping one of the problem sites.
If the DNS is working and the site is alive, you'll get something like this:
Pinging example.com [220.127.116.11] with 32 bytes of data:
Reply from 18.104.22.168: bytes=32 time=291ms TTL=46
If there is no such site or if the DNS has crapped out, you'll get something like this"
Unknown host fhetebn.com.
If the DNS and the site are working, it may simply be timing out.
If at first you don't succeed, you're about average.