I noticed that, since some weeks, I can't anymore post in this forum using linux OS.

Using Windows and firefox browser i have no problems, while using linux and firefox i get this error everytime i try to submit a message or a PM:

Warning: your browser doesn't send the HTTP_REFERER header to the website.
This can be caused due to your browser, using a proxy server or your firewall.
Please change browser or turn off the use of a proxy
or turn off the 'Deny servers to trace web browsing' in your firewall
and you shouldn't have problems when sending a POST on this website.

I tried to check if proxy server was activated but it wasn't and, anyway the firefox browser i am using have the same settings both in windows and linux.

Any ideas to solve this?

Did you install addons like noscript or anything that can modify the headers?

You can try launching firefox from the console and see if it retrieve any error while trying to post.

Did you try launching firefox in safe mode? Try firefox -safe-mode into the console (or the binary file directly /usr/lib/firefox/firefox-bin -safe-mode .

I tried everything you said, but no infos on shell and safe mode not working too

Type in about:config into the adress bar. Check the setting for Network.http.sendRefererHeader

It should be 1 or 2.