As we announced last year, Firefox plans to drop support for all NPAPI plugins, except Flash, in March 2017
Mozilla Drops Support For NPAPI Plugins In Firefox 52
The most common way to embed the digital signing function into the website is to use NPAPI plug-ins.
It means that with Firefox 52 you won’t be able to use hardware signature tokens in the usual way.
Examples of errors
This plug-in is not supported
NPAPI plugin is required by this app
The applet couldn't start
The browser does not support NPAPI plug-ins
This site uses a plug-in that is unsupported
Couldn't load plug-in
Browser Digital Signature Kit includes a special Firefox extension and a plug-in. Once the user installs this software your website can search certificates and sign data in a way you need it