You can hit the stop button at the end of the awesomebar. If that button is not there, you can reset the toolbar layout.

• See the Navigation buttons like back, home, bookmarks and reload are missing article

If you have problems loading pages you should see the Websites show a spinning wheel and never finish loading article.

At the right end of the address bar is an icon that changes between stop ("x"), reload (circle-arrow), and go (if you are typing in the address bar). When the page won't finish, if you see the stop button, you can try clicking that.

In some cases, the page is done loading, but an image or video is not loading. I'm not aware of a specific way to stop Firefox from trying to load those, but sometimes pressing the Esc key might help.

If there's a particular page that has this problem that you would like feedback on, could you post the URL?