It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Thank you, Fred. I'm afraid I already tried that. Turning off acceleration didn't work, and disabling all the add-ons didn't work either. The only thing that worked was starting in safe mode--and that's true even for a profile that's completely clean, no add-ons at all. The one exception to "clean" is that it seems that Shockwave Flash is installed (by default), but it's not activated. I didn't want to remove it, as it seems to be part of the Firefox installation.

Flash is a plugin, not a part of the browser. The browser makes use of it when called. Your system details shows flash is up to date.

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

You can check the Web Console for possible messages about blocked content.

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. You can click shield icon for more detail and possibly disable the protection. You can check the Web Console for messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks, Cor-El. I'd tried all of those things (except removing Enhanced Tracking Protection) before I posted my plea; unfortunately, disabling Enhance Tracking Protection didn't work, either. Again, the only way I can get the radar to display is to start in Safe Mode; disabling add-ons, clearing caches, deleting cookies, etc--all the things listed on the Mozilla help pages, doesn't work. Only running in Safe Mode does, even for a clean install with no cookies, history, add-ons, anything else.

FredMcD: You say "Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings,..." I guess that "resets some settings" is what's going on here, because the radar displays when in Safe Mode, but I can't get it to display when I turn off everything I can turn off. It won't even display on a clean install, with no history, no cookies, no add-ons, unless I run in Safe Mode. I would guess that when Firefox runs in Safe Mode, something that I cannot access is not being implemented, and that something, when implemented in normal run mode, is stopping the radar from displaying. Is there anything else I can try?

Are there messages in the Web Console about issues with WebGL or about blocked content or otherwise related to this issue ?

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

Cor-El:

I'm not familiar with Web Console but, from what I can see, there are no messages about WebGL or blocked content. The only entry is:

Security Error: Content at moz-extension://9287c312-a014-49c9-b8bb-f7b193cee72f/ may not load data from blob:null/27af4e20-043c-4434-a7e7-54ad7c8c7478.

I don't know if this is relevant.

That is a problem with an extension.

You can check the UUID on the about:debugging page to see what extension this is about. See the "Installed Extensions" section.

• https://developer.mozilla.org/en/Tools/about:debugging