Adobe Flash installed by flashplugin-installer (27.0.0.183ubuntu1) and working with Firefox (56.0). Not working with Epiphany (3.26.1), Surf (2.0), or MiniBrowser (libwebkit2gtk-4.0-37). Tested with http://www.speedtest.net ("legacy" site), http://get.adobe.com/flashplayer/about, and others.
Can you confirm that you have the WebKitPluginProcess2 binary installed? It's a subpackage on many distros. It should be in the same place as WebKitPluginProcess (pkglibexecdir, not completely sure where that is on Ubuntu).
WebKitPluginProcess2 was installed. Purged and reinstalled libwebkit2gtk-4.0-37-gtk2. Flash started working! Unfortunately, the reinstall broke https://boltbus.com so bookings couldn't be made. Uninstalled libwebkit2gtk-4.0-37-gtk2 to it. I believe boltbus.com works without flash if not installed, but breaks with flash running through WebKitPluginProcess2. Works as expected in firefox.
(In reply to bugzilla-ok from comment #2) > WebKitPluginProcess2 was installed. Purged and reinstalled > libwebkit2gtk-4.0-37-gtk2. Flash started working! OK....