Since support for 10.6-10.8 was dropped we can remove special code to implement custom fullscreen logic for 10.6 users which didn't have native fullscreen. Some things I saw are: * https://mxr.mozilla.org/mozilla-central/source/browser/themes/shared/browser.inc#7 (#restore-button) * https://mxr.mozilla.org/mozilla-central/source/browser/base/content/browser.xul#976 but there is probably other places and it would be worth auditing browser-fullScreen.js.

Pushed by mozilla@upsuper.org: https://hg.mozilla.org/integration/autoland/rev/7b2be27b7bb9 part 1 - Make resizable window on macOS always show full screen button. r=spohl https://hg.mozilla.org/integration/autoland/rev/6d7fac080dca part 2 - Remove nsIWidget::SetShowsFullScreenButton. r=spohl https://hg.mozilla.org/integration/autoland/rev/d809b87c8868 part 3 - Remove OS X detection code from full screen handling. r=dao https://hg.mozilla.org/integration/autoland/rev/ea043dfff6a6 part 4 - Remove now-useless fullscreenbutton attribute. r=dao