The connection has timed out

I 've a problem with firefox portable while using it on my computer.
Cannot load any page.
I 've used it on my school's computer and there is no problem.
I use a DSL connection.
In the "how firefox connects to the internet" field it's chosen "direct connection"
(I 've tried to change this but with no response).
I use the Zone alarm antivirus.
The application is installed on a flash drive but I 've tried to use it installed on my hard drive and the same problem appears.
(I have no problem using thunderbird through my flash drive and i have no problem using the original "not portable" version on my computer)
I 've tried reinstalling it but the problem remains.
Please help!!