Playing videos breaks after reloading pages in ESR 78-based builds
We set `browser.privatebrowsing.forceMediaMemoryCache` to `true` in
#33856. It turns out that this caused a
[regression](https://bugzilla.mozilla.org/show_bug.cgi?id=1650281) which
lead to videos breaking if one re-loads pages (see [comment
3](https://bugzilla.mozilla.org/show_bug.cgi?id=1650281#c3) for steps to
reproduce).
Lucklily, this is already fixed in Firefox 80 and the patches are small.
We should backport them.
issue