Sometimes Firefox shows only a overlay with a loading animation. On dev tools you can see the page you wanted to load exists behind that overlay. It seems this is a bug in Firefox because it also happens when addons are disabled. Do you experience this too? It's rare, but when it happens, you can't get rid of it. In my case it happens when I open a bookmark e.g. a known search engine. Reload page has no effect. At some point during reload it goes away. But I can't reproduce it.
