Comment 24 for bug 1659922

@dupondje Not exactly: In my case FF was broken directly after the update to 51.0.1, without any further change, including configuration and extensions/plugins.

And yes, by its nature, it's a regression. Not in FF upstream, but in the ubuntu package (apparmor profile).

The problem itself could be older: I have seen this issue earlier (FF49/50), when playing around with E10S manually (in about:config). Up until FF50 that could be worked around, but my workaround (installing extension "ubuntu modifications") does not work any more for 51.0.1.