Append this line of text to the URL of whatever Flash game you're on: "/format/flash?emulate=flash"
It might not work because it's only emulating Flash stuff and I think it only normally shows up if the game is properly supported, but anyways it was worth a try.
Anonymous-Frog
Newgrounds uses Ruffle, a Flash emulator that primarily supports AS2 (ActionScript 2) submissions (usually made before 2006, although plenty of stuff after that are also supported).
It doesn't currently support AS3 yet, which a lot of newer Flash games use.
You can also enable Ruffle by ticking 'Use Ruffle Flash emulation, if possible' in the Account Settings.
Alternatively, you can download the Newgrounds Player if Ruffle doesn't currently support a certain submission. Tick "Use Newgrounds Player for Flash" if you want this to be the default.
https://www.newgrounds.com/flash/player