This is something weird, Flash Player stopped working in Firefox in UM 18.04. It was working normally until I installed Chromium and installed Flash Player for Chromium, which I do not exactly remember how I did, it was just a copy and paste from a Google search.
In Firefox all seems to work as expected: when flash is detected it shows a clickable icon to activate flash, so I do and supposedly it will start playing. But then nothing happens, either it does nothing or it freezes the tab or the entire browser.
And the same web pages work just fine in Chromium. As well as in Firefox using an installation of Linux Mint MATE 18.3 I have on this laptop, based on 16.04.
This is the output of Firefox ran from the terminal when trying to rum Flash Player content:
[code]$ firefox
(firefox:5666): LIBDBUSMENU-GTK-CRITICAL **: 18:36:29.041: dbusmenu_menuitem_property_set_shortcut: assertion 'gtk_accelerator_valid(key, modifier)' failed
[Parent 5666, Gecko_IOThread] WARNING: pipe error (131): Connection reset by peer: file /build/firefox-ikZgcS/firefox-59.0.2+build1/ipc/chromium/src/chrome/common/ipc_channel_posix.cc, line 353
NOT SANDBOXED
[fresh 5837] not implemented: PPB_OpenGLES2VertexArrayObject;1.0
[fresh 5837] not implemented: PPB_OpenGLES2DrawBuffers(Dev);1.0
[fresh 5837] not implemented: PPB_BrokerTrusted;0.3
[fresh 5837] not implemented: PPB_AudioOutput(Dev);0.1
[fresh 5837] [PPB] {zilch} ppb_network_monitor_update_network_list
###!!! [Parent][MessageChannel::Call] Error: Channel error: cannot send/recv
###!!! [Parent][MessageChannel] Error: (msgtype=0x59000A,name=PPluginModule::Msg_PPluginInstanceConstructor) Channel error: cannot send/recv[/code]
These are all the packages I have installed using the search term "flash" in Synaptics:
Already messed around a lot trying to get Flash Player to work again on Firefox but without any luck. How can I fix this?
edit: the issue remains with a clean profile; also remains in the latest Firefox beta 60.0b16.