If BOTH browsers display the same behaviour it suggests that there's a problem with the way that your computer connects to the internet. Your problem sounds like a recurrence of a DNS fault which was widely reported here on AB a year or two ago.
Try accessing your router's settings and looking for the 'DNS' information. (If you don't know how to get into your router's settings, please post again to tell us which make it is). It will normally show that your DNS server is 'automatically assigned'. Click on the option to change that, so that you can enter the primary and secondary addresses yourself. (Don't worry if that sounds complicated. DNS addresses are simply strings of four numbers, such as 195.8.69.7). Then enter the two DNS addresses used by your ISP. You can find them on your ISP's support pages or (for most UK ISPs), here:
http://www.kitz.co.uk/isp/dns.htm
(If in doubt, simply enter the 'OpenDNS' addresses, at the foot of that page. They'll work with any ISP).
I'm not guaranteeing that following those instructions will work but I've fixed similar problems, both on my own PCs and on those of others, by changing the DNS information. What I can guarantee is that it won't do any harm to switch from 'automatically assigned' to 'user assigned' DNS addresses, so it's worth a try.
Chris