I found more solutions, but no one worked for me.

This is for windows probably: https://support.mozilla.org/en-US/questions/1100682

Then I found solution by setting browser.autofocus to false in about:config ... but the keyboard still pops up...and it is verry annoying, because I like this browser the most.

If you are bothered by this because you have a physical keyboard, then switching your default keyboard to 'Null Keyboard' might be what you want. It applies globally, not just to firegox though, obviously.