Firefox — Notes (52.0)
Removed support for Netscape Plugin API (NPAPI) plugins other than Flash. Silverlight, Java, Acrobat and the like are no longer supported.
That’s really, really bad. At this point Firefox was the only browser which was supporting Java on Linux. I am forced to stick to the 51 version (52 64bit ESR does not support Java also).
You may ask, why on earth do you need Java inside your browser in 2017? My company, which does not support Linux, uses Java applet to create VPN connections to the client’s networks and Firefox was the only choice.
52 64bit ESR does not support Java also
This isn’t true on Windows 32 bit, are you sure it’s not supported? If it was supported in 51 for 64 bit Linux, I expect it is in 52ESR.
Time for your company to port to Java WebStart.
Aren’t there add-ons that can use a VPN connection?
Java was never supported on 64bit Windows. It was only ever Silverlight and Flash.
Yep, that was weird. I’ve installed 52ESR 64bit and it told me that Java is not supported.
This Java applet is not creating VPN for
Firefox only, but opens a VPN connection system-wide. I need it for my other services like Git or maven.
…I never said it was.
Feel free to reread. 
Windows also supports VPN connections, can’t you use that?
Ah, misread the original to be talking about Windows and not Linux. My bad.
No. This Java applet is the only way to connect to the client’s network. On Windows I could use also IE to do this, but I’d prefer to avoid this ridiculous OS.
I find it strange that only the Java applet would work. Windows has native support for it.
That’s in-house VPN tool, so that is the only way.
That’s Secure Application Manager from Pulse Secure. I’ve tried using openconnect to connect to the VPN but with no success. 
have you tried setting up and going through OpenVPN?