I'm using FireFox 53.0.3 on system Win7 64bit.
After AdobeUpdater noticed me about new version I opened the adobe webpage (for safety reason) and started update manually.
I followed the ErrorPage and tried installing the NPAPI-Version für FireFox manually. Installation worked till 100% but then the setup window freezes.
Then I've found this link: Flash Player Help
After click on 'CHECK' I now get the message that latest version is installed...
now I'm confused what happened..
Since the installation window froze at 100%, your installation had completed by that time. This is the reason when you click on Check button on https://forums.adobe.com/external-link.jspa?url=https%3A%2F%2Fhelpx.adobe.com%2Fflash-play er.html page you see that your version of Flash Player is latest.