Have you tried safe mode?

.....

It works for me with Adblocker Plus still on.

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

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, 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

Have you tried safe mode?

No I was hoping there was some way to avoid that (I have about seven add-ons and don't have an SSD yet so rebooting takes a long time). But I'll go that route eventually if I have to.

It works for me with Adblocker Plus still on.

The problem comes when you try to submit the question (I was able to type in my info as well).

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

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, 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

I had the built-in tracking turned off (shield is gray in my capture) but I had DuckDuckGo PE and I turned it off and it worked!

Yeah, we thought something was blocking which is why if you tried safe mode and it worked, then it had to be an add-on. Glad you found it.

Note that if you won't see an active (purple) shield when content blocking extensions are active as they take prevalence, so Firefox doesn't see this content. If you disable these extensions then it is still possible that Enhanced Tracking Protection kicks in and blocks content that was previously blocked by an extension.