I understand that Firefox is crashing using the Flash plugin and that there is an issue with "not accepting amd64 architecture"

Please make sure of the following:

1. Some problems with Flash video playback can be resolved by disabling hardware acceleration in your Flash Player settings. (See this article for more information on using the Flash plugin in Firefox).

To disable hardware acceleration in Flash Player:

1. Go to this Adobe Flash Player Help page.
2. Right-click on the Flash Player logo on that page.
3. Click on Settings in the context menu. The Adobe Flash Player Settings screen will open.
4. Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel.
   
   The image "fpSettings1.PNG" does not exist.
   
5. Remove the check mark from Enable hardware acceleration.
6. Click Close to close the Adobe Flash Player Settings Window.
7. Restart Firefox.

This Flash Player Help - Display Settings page has more information on Flash Player hardware acceleration, if you're interested.

Next Try: Adobe Flash plugin has crashed - Prevent it from happening again

https://crash-stats.mozilla.com/report/index/b9fefb3c-cd6a-461d-a10d-2b8db2150612

This is strange. Your System Details shows;

Installed Plug-ins

Shockwave Flash 18.0 r0

But the crash report also shows; FlashPlayer-10.6

These were also flagged;

libsystem_kernel.dylib =  ? Word for Mac

CFNetwork = CoreServices umbrella framework

Flash: Grab the uninstaller from here: Uninstall Flash Player | Windows Uninstall Flash Player | Mac

Then reinstall the latest version.

Flash Player Version 18.0.0.160
https://www.adobe.com/products/flashplayer/distribution3.html

FredMcD, thanks for your suggestion. I think it has worked.