I don't like native shortcuts for "Show Next Tab" and "Show Previous Tab" in Safari, I thought that Fn would be great for previous tab and Left Control for next tab because I don't use that keys in Safari at all, however I wasn't able to set that keys as shortcuts in Setting>Keyboard>App Shortcuts, how can I do that?
Both the control key and Fn key are modifier keys, which generally aren't intended to trigger anything by themselves. I think that even a macro program (e.g. keyboard maestro ) probably won't trigger on just a mod key. I know that Alfred won't trigger a workflow on a mod key.
There is a program called Ukulele that lets you edit your keyboard layout, and that would make it possible to remap your keyboard's behavior to your heart's content, but either you'd need to live with those keys being remapped all the time, or you'd need to select the alternate keyboard you created when you're in Safari.
Long story short, I think it's possible but will take some work and be annoying.