Statement from another site (
http://www.oldschooldaw.com/forums/index.php?topic=27.0):
There were 3 Studio 5 ROM releases:
10/11/91 - original release: This release did not support networking.
10/13/93 - LX release: This release supported networking. Much code was optimized resulting in better performance, and numerous bugs were fixed.
8/15/95 - LX maintenance update: This release fixed a few minor bugs: One was the 100 protocol numbers bug.
(click link above for more)
Btw.: They already added a related firmware download link as well from their site some days ago, especially after they realized that mmtuk provided his ZIP archive here at this place

As we all know, Opcode was killed later on by Gibson.
I'm very sure that the 1995 maintenance release was the very last firmware officially released for the Studio 5 LX, as I've been looking for that ROM dump for years now.
Additional hints for firmware programming & device setup:Two pieces "27C256" EPROMs are necessary. Of course you can also use so-called OTP (One Time Process) PROMs of same type in case you don't intend to erase/re-use these memory chips later on for other purposes. At least OTP-PROMs are a little bit cheaper than EPROMs.
Another option is to use "27C512" EPROMs/OTP-PROMs instead (if you want to waste more space/money). In this case two jumpers on the Studio 5 LX mainboard must be set differently. Positions are clearly depicted on the board, so nothing can be done wrong.