[xpsycho01x]

Sometimes when I go on some website, then it recognize my operating system ( linux ).
Is there a way to hide my real operating system ?

[Black‌jack]

https://addons.mozilla.org/en-US/firefox...-switcher/
https://chrome.google.com/webstore/detai...pkjnoahfmg

Change your user agent to a Windows one.

[WojeT]

You also need to disable Javascript, get noscript as well as doing what Blackjack suggested: https://noscript.net/

[Betty Blue]

If you're worried about Firefox leaking information you should also disable WebRTC.
In about:config change media.peerconnection.enabled to disabled.