I know my english is bad, so i'll try to be really clear with short sentences
Sorry for that.
Comodo and Avast are installed on this PC from.... 8 years, i guess, and always kept updated.
Comodo is'nt blocking anything from Avast: i set it as "trusted" at that time and never changed it.
The problem appeared in the last month or so. Never had it before.
I struggled a lot trying to find the cause (removed addons on firefox, checked the router, etc) until i've disabled the active protection for 10 min for a faster file moving.... and all FF tabs suddenly were reloaded.
The problem appears more times in a day, at random intervals (a matter of hours).
Not all the sites are blocked during the problem: some are reachable, some not. My pop3 checker always works (on multiple accounts/providers).
Suspending the active protection ALWAYS solve the problem, no matter how much time i choose for the suspension.
Between my PCs and the ones of my sons (2 win7, 3 with XP), this PC (XP) is the only one showing the problem.