Could be a problem with third-party cookies.

Try to create a cookie "allow for session" exception (http://steamcommunity.com) to see if that works for you.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

You may have to remove existing cookies.

You can inspect cookies for the domain in the currently selected tab.

• click the "Site Identity Button" (globe/padlock) at the left end of the location/address bar
• click the arrow to expand the security message
• click "More Information" to open "Tools > Page Info"
• click "Permissions" to inspect and modify permissions
• click "Security" to inspect and modify cookies in a standalone window

Thank you cor-el, your suggestion fixed the problem. I really don't understand how this could occur, nor how I could have figured it out without your kind guidance.

When I was looking for a solution before coming here, I was told that turning off the "media.mediasource.whitelist" preference did the trick. However that preference seems to be deprecated in FF44 64bit. So my last question would be: is anything in Firefox acting as this function did on previous versions, and if yes, how can I turn it off (usually I hate when Firefox keeps me away from content "for my safety").

Best, N.

You're welcome.

The Web Console (3-bar Menu button or Tools > Web Developer) showed a security error message for me. That is why I checked for cookie issues because I've seen more such cases with embedded videos. I don't know if this is a change in Firefox, but you wouldn't get this problem if third-party cookies are enabled (this is the default).