Browsers should have one built-in navigation transition. When you click on a link that leaves the page, something should happen immediately. It may take seconds for the new page to load, especially on cell networks. There's no immediate user feedback, resulting in multiple clicking, which restarts the page load. So on page exit, something visible should happen. Dim out the page, zoom it down to a tiny rectangle, swoop it off screen and replace it with a busy icon, or something.
(Unfortunately, you can't implement this with a Firefox add-on, short of patching every link in the DOM, or I would.)
> When you click on a link that leaves the page, something should happen immediately.
On every browser I use, the UI changes immediately after clicking a new link.
Pay attention the next time you click a link that's not in cache. Look for messages in the status bar (which is usually hidden, but typically becomes visible on resource load or when hovering over a link), changes of favicons to spinners in tabs, and the change of the reload graphic to a "stop" graphic.
All of these UI changes are obvious to me.
I get that some people are impatient, but these same impatient people likely know how to and are licensed to operate a motor vehicle. Motor vehicle operation is a task that requires constant attention and much finer attention to detail than is required to notice the current browser UI cues that indicate that a new page is on the way.
Here's Firefox, immediately after entering a site name into the URL window.[1] The only visible effects are that the tab name has changed to "Connecting", and the reload arrow has change to an "x". On Firefox Mobile, you don't get either of those, just a one pixel high progress bar at the top of the screen. It's worse on mobile, where things are slower and there are fewer auxiliary GUI items on screen.
At this point, you've only partially left the page. Some clickable items on the page being exited will still work. It's not entirely clear exactly when event processing for the old page stops.
Those are things that happen to the UI immediately after asking it to navigate to a site that you typed in.
Now that you've demonstrated the UI changes that happen immediately after you type in a URL and press Enter, what UI changes happen when you click a link that leaves the page?
That's the complaint of yours that I was addressing in my comment. :)
(Unfortunately, you can't implement this with a Firefox add-on, short of patching every link in the DOM, or I would.)