The Mac NPAPI Flash sandbox is enabled on Firefox Nightly today in the shape that we plan to ship in Firefox build 62. It was first enabled on Nightly in April and has had a few updates and fixes since then. Some fixes have not been ported to Beta yet, so we recommend using Nightly at this time.
Disabling the sandbox requires using about:config to set "dom.ipc.plugins.sandbox-level.flash" to 0 and then restarting the browser. The sandbox is known to prevent the following features from working and we plan to publish an article on support.mozilla.org documenting the limitations and how to disable sandbox.
Print to PDF from the Flash print dialog
Print to “Open in Preview” from the Flash print dialog
Quicklook in the File->Open dialog
Saving files to the filesystem from a Flash applet
Flash applets downloading and running the Adobe Air installer