Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Perfect! Thanks cor-el: Good instructions. It was the add-on extension FlashBlock, which makes some sense, in that what wasn't happening with the street-level feature was that I wasn't getting the image of the street. Cannot now reconstruct what led me to <https://support.mozilla.com/en-US/kb/Troubleshooting+extensions+and+themes>, but your instructions got me there, and following the trouble-shooting steps narrowed down the problem, the way one used to have to do with Mac extension, identify the culprit by process of elimination and going first for those that might make sense, as FlashBlock did to me, whether for the right or wrong reason.