Mozilla decided to stop supporting NPAPI plugins for Firefox browser two years ago. NPAPI plugins are binary components integrated to the browser. Some well known NPAPI plugins are Flash, Java, and the GNOME Shell web extension.
The stock Firefox 52 browser disables these components. The exception is Flash, but Firefox will remove its support in the future, too. Fortunately, Fedora users still have options to bypass that plugin ban.
Enable plugins in Fedora Firefox browser
This may be the easiest choice for now. Firefox 52 allows you to override the plugin block using a preference setting in about:config. In Firefox, enter about:config in the location bar, and confirm the warning dialog. Then right click anywhere on the option list and add a new Boolean option named plugin.load_flash_only and set it to false.
Be aware this entry could be removed anytime without further warning, and is strictly unofficial.
Install Firefox 52 ESR
This is the official to run NPAPI plugins by Firefox now. However, at the end of this year Firefox 52 ESR (Extended Support Release) is planned to be discontinued.
The Firefox 52 ESR binary package is provided from Mozilla site. Download and unpack the tar archive somewhere in your home directory and launch the browser. You can then register it as a default browser. It also updates itself automatically when a new release is available.
You can have more than one browser installed. To choose the default in Fedora Workstation, go to the Settings control panel, and select Details, Default Applications.
Use dedicated browser for plugins only
This will be the final option when Firefox 52 ESR reaches its end of life. You can still use an old and unsupported browser, but it may contain flaws and vulnerabilities which won’t be fixed. A typical use case may be an internal system which needs a Java plugin but is otherwise secured. Never use an old browser for public web pages, since the browser may have known, exploitable flaws.
You can use a different browser profile for such a dedicated browser. Open the Terminal app, change directory to the old browser, and run this command:
./firefox -ProfileManager -no-remote
To launch the browser:
./firefox -P your_dedicated_profile -no-remote