Yes, that is annoying. It might have something to do with the bottom panel. What happens if you enable/disable autohide on that bottom panel? What if you move it to the left side of the screen instead of the bottom? What if you make it larger/smaller?

Basically, what happens to the size, position, and existence of the gap when you do these things.

Despite that if I have autohide or always show panel it do nothing... the same issue.

if I drag and snap window on left or right its nicely fill desktop. But if I only click maximize button it make the gap again.

It's ONLY firefox issue... everything like Chrome, file manager or other software maximize nice.

Have no clue.

Sorry, that wasn't clear. I wanted to see if moving the bottom panel to the left side of the screen would change anything.

If all the other gtk3 apps work fine, it's hard to guess what the issue might be. Your screen resolution isn't unusual either.

I move panel right, left and up.

Nothing change. Firefox still have problem to maximize correctly .