Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars or press f11 button on keyboard if you mean website full screen.
i've added a screenshot with an arrow to the "fullscreen" icon.
That would be optimal for newbie usage.
Mouse and keyboard usage.
I like to surf on fullscreen with Firefox more website presentation without disturbing (browser elemtents and taskbar) areas that taking my point of view.