Custo mfirmware by Colin Fraser - http://www.colinfraser.com/jx10/jx.htm

This is the latest version, based on the v2.3 JX10 firmware.

The bulk of the code is copyright Roland Corp. I only provide the image as a full dump rather than a patch file to save the hassle of reading then patching an existing ROM.
You may not download this file unless you are the owner of a Roland JX10 synthesizer.

The binary image should be written to a suitable 32k EPROM (27C256) and swapped for the original firmware on the assigner board - the one with the 6303 processor.
You could always erase and re-write the original EPROM, but back it up before you do.
Details of how to swap the ROM over are here.

This firmware allows editing of the currently active tone via midi. That is, if you are using Dual or Split mode, the Tone that is currently active for editing (the one with the dot by the number) will be modified by the SysEx.
The command format is the same as that used by the MKS70 rack version of the JX10.
For further information, see the links to other notes below.

It does not support the MKS70 single patch/tone dumps nor does it support sysex editing of patch parameters.
I have been using the Emagic SoundDiver MKS70 editor with my JX10 for tone edits, and it has been working well.
I considered implementing direct editing of the Upper and Lower tone without having to switch the selected tone from the front panel, but as my intention was to allow a midi fader box to be used for editing the JX in the absence of a PG800, I decided not to bother.
I believe Tone B editing is broken on the MKS70 anyway.