It sounds like a problem with an add-on, in particular the Forecastfox extension. To test this try disabling the add-on in safe mode, for details of how to do that see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Yes this worked, but I had tried starting Firefox in Safe Mode before, entering exactly (copy & paste) as instructed by Firefox (firefox -safe -mode). It failed to open Firefox in Safe Mode. With some experimentation, I discovered that the problem is putting a space between the word firefox and the dash. The correct entry is to run "Firefox-safe-mode". Firefox troubleshooting should correct that minor typo. Once Firefox was finally in safe mode I was able to disable the Forecastfox add-on, which I had already suspected as the problem. Thank you for answering my question.

You're welcome

I checked the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and safe mode articles, both have the space between Firefox and the dash.

There only need to be a space between firefox and the first dash (firefox -safe-mode), there should be no space in -safe-mode

Apparently, at least in my system, putting a space anywhere in the syntax will not work to open Firefox in safe mode. I just tried again. A space between firefox and the dash will simply open Firefox in standard mode; without the space, Firefox correctly opens in safe mode. Perhaps if it doesn't make any difference on most systems, the Firefox articles should omit the space completely or suggest trying both alternatives. Frankly, I don't see how my system is any different than any other generic Vista system, as it's basically a standard factory setup. But I suppose it's possible. Thanks for your feedback.