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In this post I look rather for clarification than technical solution (the problem is already solved, see below).
I just upgraded from 14.04 to 16.04 and installed a fresh Firefox, which happened to be Quantum 57.0.3 (64 bit). Flash didn't work, thus I installed
flashplugin-installer, which has in its description:
The Adobe Flash Player plugin will work with a range of web-browsers >including, limited to: * Firefox
Unfortunately it didn't help. What helped was the installation of
browser-plugin-freshplayer-pepperflash instead. The description of the latter package states:
The main goal of the project is to get PPAPI (Chrome) plugins working in Firefox (and any other web-browser supporting NPAPI plugins
I wanted to understand why the former package didn't work, thus I started to read and found out here that Firefox since ver 52 does not support NPAPI plugins, except for Adobe Flash. Therefore, it seemed that having
adobe-flashplugin installed should be enough and one does not need to deal with the NPAPI/PPAPI issues. Indeed, after I removed
browser-plugin-freshplayer-pepperflash and sticked to
adobe-flashplugin, everything worked fine. Thus, I suspect that the crux was in one of the packages required by
browser-plugin-freshplayer-pepperflash, not that package itself.
Would the conclusion be that Firefox in 16.04 ships without Flash support, even when
adobe-flashplugin is present (I wonder why)?
If so, some users might bump at this problem, just as I did.