"Only a single instance of this application can run" error when installing Flash

I deleted Flash Player when trying to solve a Shockwave Flash Google Chrome issue, and after being unable to solve it I switched to Firefox. I now can't get the installer to open. I double-click on it, it asks me if I want to run it. I click Yes, then the window closes and the installer doesn't open. When I try running the installer again, I get the error message "Only a single instance of this application can run". I have tried everything. I closed Firefox, restarted my computer, and ran the Flash Player uninstaller. I am running Windows 8.1 64-bit with Mozilla Firefox and I am trying to run the latest version of Flash Players as of October 26 2014. Please help!