Add item to Security & Privacy > Privacy > Accessibility
Try dropping /Applications/Firefox.app/Contents/MacOS/plugin-container.app to the preference pane instead.

When I tried opening a page that uses Flash in Firefox, ps -ef|grep -i flash contained a line like 501 28715 28710   0  4:44AM ??         0:01.01 /Applications/Firefox.app/Contents/MacOS/plugin-container.app/Contents/MacOS/plugin-container /Library/Internet Plug-Ins/Flash Player.plugin -greomni /Applications/Firefox.app/Contents/MacOS/omni.ja -appomni /Applications/Firefox.app/Contents/MacOS/browser/omni.ja -appdir /Applications/Firefox.app/Contents/MacOS/browser 28710 gecko-crash-server-pipe.28710 org.mozilla.machname.157869410 plugin.

If others search for how to add applications to the list with a script, see http&#x3a;//apple.stackexchange.com/questions/111903.