hi,,, your internet connection is good if you receive mail,,,, next time it happens open command prompt and send a test ping to google url to test it,,, in uk its www.google.co.uk,,, if it returns ok then i would start to look at the connections into and out of the pc and sockets,,, make sure all filters are ok and that everything going to a phone socket is filtered,, if all is ok in this department,,, then next,, the browsers and your security,,,, uninstall FF and download a clean addition,,, use revo to uninstall it,,, switch off all add ons and browser helpers,, tool bars etc,,, check the browsers security is not too high or blocking anything,,, then lastly check your firewall is allowing both browsers through,,,, cant give much direction on IE as i don't use it, so will leave that to you or someone else,,,,, if all checks out good then contact your isp and ask them to check your line,
hope this helps and
good luck.
also as an after thought,,, are you wireless ??? if so then use an ethernet cable from the pc to the router to test the connection is good,, if all works fine then your wireless is playing up, which we can go into later.