In an effort to streamline Firefox's UI, the latest Firefox nightly builds have moved the Stop/Refresh button into the right corner of the Location Bar. I like this move even if the Firefox developers are simply copying design ideas from other browsers (in this case, I believe Safari was first).
When you're typing in the Location Bar, a green Go button is shown:
That switches to a red Stop button when the page is loading:
The thin line you see in the location bar is a progress indicator that indicates how much of the current web page has loaded - I don't like the way it looks and I think it'll probably change in future.
When the page has fully loaded, it changes to a Refresh button:
This leaves a minimalist and compact toolbar:
I should actually remove the Home button - I don't ever use it.
To try out the new stuff coming in Firefox, download a Firefox nightly build now.