This may be your problem,

Firefox v52+ Releases disables all plugins except for Flash Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work? {web link}

FredMcD said

This may be your problem,

Shockwave Flash is old name for Flash Player so no it s not related.

You can check the handlers.json file in the profile folder.

You can also check all involved MIME types in the OS.

• xdg-mime query default <MIME type>

Thanks everyone. I should have said that I'm aware that newer versions of Firefox block the Shockwave plugin by default; I have already taken care of that. This plugin did work before, it's just that the mime type which is assigned to SWF file type does not use it.

xdg-mime query shows that indeed all .swf files are assigned to mime type application/vnd.adobe.flash.movie. It seems the trouble had started with installation of a new video application, which might have messed up file type.

So the question is really how to restore the association of .swf file extension with mime type application/x-shockwave-flash, and / or how to convince Firefox to use the plugin for all mime types which are associated with SWF files?

Thanks much.

Found the answer here Basically, it requires changing the mime type in /usr/share/mime/packages/freedesktop.org.xml