Earlier today I experienced that my flash plugin no longer was working. Tried several measures, but no success. I was just about to post a message on this site if someone else had experienced the same (under FireFox 31.8 ), when I remembered the excellent search facility on this site. Hence, I searched for flash, and then there was an earlier post from NeillWaldauer (and a most helpful response by Dave Yeo to remove pluginref.dat in the FF profile directory). Voila, flash was back and up running.
The morale: while it is easy to ask for assistance on the forum, it is even easier and faster to make a search (go to the forum pages and search from there), and it may just happen you find what you are looking for). In this case it did. So, many thanks from Norway to those administering the forum

.
Trying to recover my steps and reasons this error occurred almost out of the blue, I recalled that I had installed the "Copy plain text" add-on. I am not certain this is the reason flash had been moved in pluginref.dat to the invalid segment of that file, but the time the file had changed, corresponds well with when flash stopped working.