I don't think Firefox supports Java portable, and I'm pretty sure it can't (legally) support it... sorry.
Mozilla Firefox, Portable Edition-Specific Issues
PortableApps.com was founded by John T. Haller, whose Portable Firefox started the portable software trend. We've been making software portable since March 2004.
For these special cases, I prefer to download and use Firefox portable with Java portable instead of a local JRE.
Firefox Portable: The launcher FirefoxPortable.exe automatically detects portable Java versions. jPortable: PortableApps.com is hosting a portable version of Java on sourceforge.net.
Firefox Portable Java. Monday, December 19, 2016 3,996 views 3.
PortableApps/CommonFiles/Java — here you have JPortable. PortableApps/FirefoxPortable — here you have Firefox. There is another way to accomplish the same task. It is useful for instance if you have dozen of portable Firefoxes installed.
Portable Firefox with Java support. User Help for Mozilla Firefox.
We are using FireFox portable and Java portable to achieve this.
Mozilla Firefox Portable Offline Installer Free Download.