Hello, is it possible from the extension to focus / select all text in address bar? Is it possible to empty address bar? Is it possible to focus webpage content instead? Please help! Thanks in advance!

You can focus stuff in a webpage, but you can't interact with the OmniBar from JavaScript.
As far as I know, most elements can be focused using element.focus();

ok i see what you mean, pity it's not doable..

i don't know exact element on the page.. i would just like to focus content control (like on mozilla where i would simply do content.focus())

Content control? You could loop through form elements or soemthing..or like document.getElementsByTagName("input"); or whatever..not sure exactly what you're needing.