If for some reason you are moving from Safari to Firefox on OS X, you might miss the Cmd-Shift-H shortcut key to go to the homepage. On Firefox, this is by default Option-Home but since there's no Home key on Apple keyboards, this becomes the Fn-Option-LeftArrow shortcut which I can never remember.
An alternative is to press Option-K, Enter. It'll focus on the search field in the toolbar, and pressing enter will take you to the search engine home page.
If you have removed the search field from the toolbar, then simply pressing Option-K will be enough. Very useful when you don't want to close the last remaining tab, but want to clear the current page. Update 2016-11-22: this last option no longer works because it'll open the options menu and focus on the search field there.