Should I raise this concern with Mozilla as a problem? If so, how do I do that?

Hi Colin

Is there any error message shown?

Hi Paul, No, the dot to the left of the tab name goes side-to-side for a while then stops, leaving a sand-timer icon but the pane remains blank. Being new to this forum, I replied to my original question, then discovered how to reply from the email you sent. In the meantime, one of my test websites did actually load - after a long delay - but was then entirely unresponsive. Thanks for your help - Colin