My latest error(S) ...

Suddenly, I have lost my "Active Desktop" (which I hadn't "lost" in years - and which I still do not understand) - anyway, additionally, it shows me an "Internet Explorer Script Error" message (though Firefox is my default browser), and which I don't know whether to click on "Yes", "No" (and I'm sure someone will come along and tell me there's a "maybe so" option)?

"Probably" related (?), is that all of a sudden, there is nothing displayed in the address bar of my Firefox browser ... oh, except that it does change to the color yellow (= secure) for those (secure) pages that I visit.

What might I have done wrong THIS time??