The adobe flash player plugin is properly installed to the variable MOZ_PLUGIN_PATH. I made sure to place the shared object for where the Gecko engine searches for plugins, as seen [https://developer.mozilla.org/en-US/docs/Plugins/Guide/Plug-in_Basics#How_Gecko_Finds_Plug-ins here]. In my case, I put the shared object in /usr/lib/mozilla/plugins and in $HOME/.mozilla/plugins. I verified the location of the variable in my shell with $ echo $MOZ_PLUGIN_PATH I also made sure to copy the usr/ folder that comes with the adobe flash tar.gz archive $ sudo cp -r usr/* /usr I start Firefox in safe made in about:support. I then went to about:plugins and the plugins sections in about:addons to verify that flash has been properly installed. Then, I test the ability to play flash on [https://get.adobe.com/flashplayer/about/ here] and on a random flash site as seen in the screenshots. In the profile I am currently using in about:profiles, I made sure that the http_https_only toggle in about:config is set to 'false'. I veryify by opening other types of local files in Firefox: pdf, mp4, etc (pdf screenshoted). I then make sure that Firefox is what is set to open files with the mimetype of .swf files via $ xdg-mime query default application/x-shockwave-flash To verify the output, I checked the entry in $HOME/.config/mimeapps.list for the entry for swf mimetype files. I cannot use the [https://www.adobe.com/support/flashplayer/debug_downloads.html flash player projector]. This is because the projector version of flash calls for the function pthread_yield(), which is a part of the pthread.h header for glibc, as seen [https://linux.die.net/man/3/pthread_yield here]; I use musl-libc for my C library and standard C headers. The call for pthread_yield() can be seen via $ ldd /path/to/flashplayer The NPAPI version of flash does no such thing. That's why I can play swf mimetypes through the flash firefox plugin. I am currently using version 79.0 of Firefox. I can't remember which version I had before, but I was previously able to play local swf files. I am not sure if an update broke it somehow. Attempting to open a swf file will simply prompt firefox for the xdg-open check for what I would like to open the swf file with over and over again - an infinite loop. I know flash is no longer going to be support by December 2020, but until then I need to finish up some things. I make this as a clarification to my [https://support.mozilla.org/en-US/questions/1297240 previous question], which seems to have died off. Perhaps I wasn't clear enough in it. The about:support I use for my daily profile can be found [https://pastebin.com/CjGwnL3T here]. If needed, I can post more info about my Firefox profile in my user. Thanks