As a rule for Firefox, I have every window automatically start maximized (not fullscreen).

The window is horizontally and vertically maximized, with the awesome panel up top.

However, if I go to YouTube and decide on making the video fullscreen, and then afterwards exit fullscreen, the Firefox window loses its maximized state.

I have searched this, but all the info I can find is about YouTube back when it used the flash player.

In summary:

Maximized Fullscreen (manually initiated by me) Unmaximized (after exiting the YouTube fullscreen video. Wish it stayed maximized like it was at step 1)

Here is my rule for Firefox: