Hi, try disabling your theme - Add-ons Manager (Ctrl+Shift+A) > Appearance. If that doesn't help, try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem. Thank you!

Fixed! I first tried disabling themes, then tried safe mode, neither of which fixed the problem. I then linked to the 'Troubleshooting extensions, themes and hardware acceleration' web page and ran the 'Refresh Firefox Feature' (https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings). Running the refresh feature deleted/removed whatever was causing the problem and mapquest now displays the map correctly. It looks like that feature is also available via a selection button in the Firefox Safe Mode window. Thank you!!