I wonder whether this is a "cache" problem. In other words, Firefox may be using older files stored in cache to redisplay the page rather than fetching fresh data. Next time you see this problem, could you try to reload the page bypassing the cache using either:

• Shift+click the Reload button on the toolbar
• Command+Shift+r

Any difference?

No difference. It's weird, because all of the other content modules on my home page are just fine (stocks, sports, email, headline news, etc), and the weather module displays normal on Safari.

But something is stopping the high temperatures and forecast from returning to the homepage. As I said, if I remove the weather module and then re-install it, it comes up fine, until I leave, refresh or act on any other part of the home page.