No, I'm not talking about Flash Player. It's definitely Shockwave Player.
The full and exact error message (in German, for I have German language settings) reads as follows:
"Warnung: Nicht antwortendes Plugin.
Shockwave Flash ist möglicherweise beschäftigt oder reagiert nicht mehr. Sie können das Plugin jetzt stoppen oder fortsetzen, um zu sehen, ob das Plugin weiter arbeitet."
Translated it should read something like:
"Warning: unresponsive plugin.
Shockwave Flash is possibly busy or not reacting any more. You can stop the plugin now or continue, in order to see if the plugin continues to work."
And yes, you are right, I only get this error message in Firefox, not in Chrome or Edge. Chrome and Edge just stop playing videos and / or crash completely (the same as Firefox but without an error message).
But every time I get this error message in Firefox, all the other browsers stop working simultaneously. So it has to do with it at least somehow.
Even the pre-installed windows 10 video player (called "Film und TV" in German) as well as every other video player program I own (e.g. Media Player, DivX Player, Power DVD for Lenovo Think, VLC for Windows Store) stops working as soon as I get this error message in Firefox and when I start to play a locally saved video. The only difference is that I don't get an error message in those players. They just don't play the video and show either a black screen or the very first frame of the video.
And when that error message in Firefox vanishes (like when I updated the videocard drivers) I can also play videos in all the other browsers and also play my locally saved videos again.
But this never lasts longer than a few days, and then the error message returns all of a sudden.
This not only affects watching any type of video. Some other functionalities on websites are also affected. Like opening drop-down-lists when choosing the cause of a cancellation of an Amazon order, for example.
I don't know if it is really Shockwave Flash that is causing the problem or if this error message is just one symptom of another, bigger problem.
But it is the only error message I get, and it coincides with the problem every time. It starts the second I get this error message.