This particular tiny splinter has really got under my skin...
When Firefox 4.0 is loading a webpage it displays a stop button that overlaps the text of the address that is being loaded. Once it has loaded, it changes to a reload symbol that sits nicely to the left of the address.
I just wish the stop button was 2 or 3 pixels narrower to create a gap here.
Edit: I’ve subsequently found out this was due to the Fission extension that isn't compatible with Firefox 4.0. Mea Culpa