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
It said it was already installed; I used install --reinstall with no luck, purged it and installed again still no luck, purged again and installed also with suggested packages but no luck, flash player won’t play in Firefox. Always the same error:
[code]$ firefox
[Parent 22670, Gecko_IOThread] WARNING: pipe error (140): Connection reset by peer: file /build/firefox-hoiE9u/firefox-60.0~b16+build1/ipc/chromium/src/chrome/common/ipc_channel_posix.cc, line 353
NOT SANDBOXED
[fresh 22833] not implemented: PPB_OpenGLES2VertexArrayObject;1.0
[fresh 22833] not implemented: PPB_OpenGLES2DrawBuffers(Dev);1.0
[fresh 22833] not implemented: PPB_BrokerTrusted;0.3
[fresh 22833] not implemented: PPB_AudioOutput(Dev);0.1
[fresh 22833] [PPB] {zilch} ppb_network_monitor_update_network_list
Try changing your software sources download location to Main server if you aren’t already using it and update again, see the guide above for more info!.
I have set main server for the downloads, I believe it was the default setting or if it wasn’t I must have changed it right after I fresh installed UM 18.04.
I would first suggest a fresh install in case you have something broken which might be easier to fix?, try the something else method if it applies to you? (On the newer installer, you can overwrite the system files and keep your data but I don't have a picture of the page sorry!).
Strangely today for the first time in many boots I did not get the emergency mode message and the boot went just fine, so for now I won’t be doing a fresh install. It leaves me just with a couple of minor issues, one being this weird flash not working in Firefox (but works just fine in Chromium) and the other the pc microphone volume control missing on the volume panel indicator (Ubuntu MATE 18.04 missing microphone volume control in panel indicator) which already happened on a previous installation of a 18.04 beta. So no big deal, no major issues.
Okay, I would try removing Chromium browser and the other Flash Player you installed with that and install Google Chrome instead! (Google Chrome is in the software boutique!):
Ok the oddness reaches its peak because that worked!
Adding to that command line I just did a
sudo apt autoremove
and then reinstalled flash
sudo apt install flashplugin-installer
Flash player worked right after and no more flash player errors.
Installed after Chrome for backup browser, tested Flash Player simultaneously on both browsers and all works well.
I have no idea what was going on since I tried a bunch of stuff installing and removing Flash Player in different ways but that’s old news now, what matters is that a solution was found.