Update: I uninstalled and reinstalled kaspersky, and the problem was present again, so in my case the problem it's kaspersky.
Then i noticed that kaspersky recommends you to install the extension on firefox (Kasperksy Protection).
I installed the extension and rebooted firefox. Now with the extension installed in firefox the problem it's gone.
So my guess it's there's a bug in kaspersky if you don't have the extension installed.
Hope it helps.
