Just came across recent notes concerning the
https "mixed-content" blocks. Not sure if that's the cause here, but perhaps?
Anyway, Rodocop had discovered that those 2 prefs are controlling those blocks:
pref("security.mixed_content.block_active_content",true);
pref("security.mixed_content.block_display_content",false);
In short it means that unencrypted elements on https-pages are blocked by default since Firefox23, for security/privacy reasons.
But it turned out that broke tons of sites, so they split it into 2 prefs, one for "active" and one for "passive" content. The "active" elements are potentially more harmful, the "passive" elements are images, audios, videos, which are only "displayed".
So the default setting in Firefox is now active=blocked, passive=allowed.
It was explained in detail e.g. on this german page:
https://www.soeren-hentzschel.at/firefox/firefox-23-kommt-mit-aktiviertem-mixed-content-blocker/