firefox small screen

I am using firefox with kubuntu and whenever I load firefox it always opens in a small window by default, and I have to click the maximize icon in the upper right of the screen to get full size back. How do you set it up so it covers the full screen when the program is launched?

 

I think this is how....
Size it what you want, and while holding the shift button, click the X. Close and reopen.

This happens sometimes with popup boxes resizing the next time it is opened.