First, download jPortable.
The trick is how to unpack it.
You must place it in the same folder, where your FirefoxPortable folder is. I.e, if the folder fro all your portable programs is C:\MyPrograms, then you should have two folder there: FirefoxPortable and CommonFiles.
The CommonFiles folder contain the Java directory from the archive.
You may put this combination to as many sub folders as you want, but FirefoxPortable and CommonFiles should also be on the same level.
For FirefoxPortable to utilize portable Java, you also need the latest launcher.