Or do you mean the floating message that describes the current activity of the browser (for example, Waiting...) or shows the URL of the currently hovered link?

That can be suppressed using custom style rules. Here is an example of a past thread on how you might do that:

https://support.mozilla.org/questions/976248

No, I was hoping to find solution for simply browsing in real full screen mode. That's mean with hidden upper part - status bar in Android which contains battery status, clock and so on..) some applications close it by default, like stock Samsung browser). Thanks for your reply.