This sounds like some kind of tracking feature. Perhaps a Google+ file is being mistaken for a video by a recorder or downloader? If you disable extensions relating to recording or downloading videos, does the message go away? You could try that here:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

More generally, when you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

This will list your youtube.com cookies, but it's possible that google.com cookies are more relevant here. You can edit the site listed in the search box at the top of the dialog to get at all the cookies you might want to clear.

Then try reloading the page. Any difference?

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.

You can't recover from this 'forget' unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Thanks to both of you. I think the problem was that my youtube and Google+ accounts had been connected. Now when the link between them is removed everything works fine.