Screen blanks/locks after 10 mins. of Flash video in Firefox

[John Hupp]

With a fully patched Lubuntu Trusty, streaming Flash video through
Firefox results in the screen blanking and locking after 10 minutes.

This sort of behavior is noted in
https://bugs.launchpad.net/ubuntu/+source/xdg-utils/+bug/1309744  and
https://bugs.launchpad.net/ubuntu/+source/xfce4-power-manager/+bug/1193716

The bugs look like duplicates to me, but developers have not deemed it
so.  There is a fix released for Bug 1193716, but only for Utopic.
There is a fix released for Bug 1309744 for Trusty, but it seems to me
that if it works, it works only for media players, not for Firefox.
(See my comment #29 on Bug 1309744.)

I have the recommended/most-recent version of xdg-utils specified in Bug
1309744.

Anyone else confirm this behavior?  If so, do you have a workaround?