Playing videos breaks after reloading pages in ESR 78-based builds

We set browser.privatebrowsing.forceMediaMemoryCache to true in #33856 (closed). It turns out that this caused a regression which lead to videos breaking if one re-loads pages (see comment 3 for steps to reproduce).

Lucklily, this is already fixed in Firefox 80 and the patches are small. We should backport them.