corrected link: https://support.mozilla.org/en-US/kb/mixed-content-blocking-firefox

You shared an HTTP address but probably you are using the HTTPS address or you wouldn't get the mixed content warning:

https://public.viavid.com/player/index.php?id=140568

That gives me a registration/login form. The image in the upper left corner is retrieved over HTTP instead of HTTPS, so that triggers the mixed display content warning on the lock. Nothing is blocked.

If you don't want the warning, and you would prefer that Firefox automatically try the image address using HTTPS, you can change this setting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste mix and pause while the list is filtered

(3) Double-click the security.mixed_content.upgrade_display_content preference to switch the value from false to true

More info on about:config: Configuration Editor for Firefox.

If you test that out, does your lock stay solid or is there still a problem when you interact with the form?

(I have a test page demonstrating the available display content preferences here: https://www.jeffersonscher.com/res/mixed-display.html)

Note: if you quote a URL in your reply, your post won't appear immediately while it passes through the link moderation queue.

Many thanks Jefferson.

Changing security.mixed_content.upgrade_display_content preference from false to true did change the lock solid grey, but didn't solve my problem with the form.

But I did find a fix: I uninstalled the extension for Firefox Relay Beta. Apparently the extension (which places the Relay icon next to email fields) is the reason I couldn't enter text in the email field. I'll let them know.