This problem is caused by a bug in Print Edit WE 27.5 which was released on 26th November.
 
A new version (27.6) that fixes the problem has been submitted to Mozilla and is awaiting review, after which it will be released, but this may take a couple of days.
 
In the meantime, as a workaround, you can download the previous version (27.4) and turn off automatic updates.