As an aside, on Win10, you're going to have a much better experience if you're using 64-bit Firefox. I believe they serve it up by default at this point, but you might want to check in Task Manager to see if Firefox says (32-bit).
You can explicitly get the 64-bit version here:
Mozilla Firefox Web Browser — Download Firefox in your language — Mozilla
Here's what I'd do:
1.) Go to Firefox > Help > Restart with Add-Ons disabled
If Flash starts working, there's a third-party plugin in play.
Disable everything and re-enable them one at a time until the problem comes back and you'll know which one it is.
2.) If that doesn't work, make sure that Flash Player is enabled in Firefox
Go to Menu (the hamburger icon on the right-hand side of the toolbar) > Add-Ons > Plugins > Shockwave Flash
Choose Always Activate from the pulldown on the right-hand side.
3.) If that was already set, do you have an off-brand security suite or software firewall installed?
If so, you may need to actually uninstall it to get Flash working, depending on the product. I don't want to call people out, but there are some frequent fliers.
4.) If not, let's uninstall and reinstall the NPAPI Flash Player for Firefox
Uninstall Flash Player - Windows: (This will uninstall all of the shared Flash Players on your system (IE and Chrome bundle Flash, so they're unaffected)
https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html
Download Flash Player using Firefox from here:
http://get.adobe.com/flashplayer/
Follow the installation instructions
5.) If Flash Player *still* doesn't work, post your installation logs:
Where do I find the Flash Player installation log on Windows?
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