PC freezing, during surfing on the internet

[Larry tb]

Was surfing on the internet using firefox.
And suddenly everything is frozen !
Trying to get control through the network but  returned "no route to 
host" :(

press reset button.

It happens each time i run firefox,  after 1-2 minutes surfing. im 
behind a proxy.

kernel 2.6.15-1.2041_FC5
firefox 1.5.0.1

Absolutly no idea what could be the cause....

larry