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?