Jump over here and you will probably find out more than you need to know. Pioneer is a very picky company and they often imply you cannot go backwards in firmware but many people have reported otherwise. Also, I could be wrong, but my understanding of firmware is that changes generally refer to burning. While it is possible that a firmware upgrade could change reading I would consider it unlikely. I'm someone will jump all over me but I would guess your problems are elsewhere.
CDSpeed is an easy to use program that will tell you your frimware version as well as many other things.