The upcoming version of Firefox blocks dangerous flash content. The Plugins section in Add-ons Manger of Firefox browser, which on Nightly has got a new option for Flash Plugin –Block dangerous and intrusive Flash content‘ with a checkbox that can be controlled by about:config preference plugins.flashBlock.enabled.
FYI, 32-bit Firefox version on Windows already offers preferences page for Flash Plugin which is to enable or disable Flash Player protected mode, with this addition, you can notice two checkboxes on 32-bit version of Firefox browser, but this is the only checkbox you’ll see for Flash on 64-bit Firefox.
FYI, the new Flash allow/blocklist feature applies to either ‘Ask to Activate’ setting or ‘Always Activate’ setting. For this bug -‘ Implement new plugin option for Flash as a checkbox to toggle the allow/blocklist feature (plugins.flashBlock.enabled)‘ target milestone has been set to Firefox 55.