Comment # 1 on bug 935445 from Thomas Mitterfellner

I think I figured out what the problem was: at some point in time I had
installed the package freshplayerplugin from the packman repo. This must
somehow have gone bad, but it was not taken up by the manual 38.0.5
installation from the tar.gz, which is why this version was working. Now that I
uninstalled the freshplayerplugin, everything seems to work fine again, also
with the FF packages from the distribution repos.

So, from my point of view, this issue can be closed.