As someone who has read the book and knows the score very well (but hasn't seen the show) maybe I can help here.
Yes, Rochester doesn't start the fire. Rochester's wife, named Bertha, starts it. When Jane enters Thornfield for the first time, Bertha has already been in the attic for a while, and is insane. The song "Poor Sister," as phantompenguinx suggested, will help illustrate this.
If you haven't read the book, its quite an interesting read. So is the book that is based upon Bronte novel, called "Wide Sargasso Sea." It details one author's view of Rochester's wife, Bertha, and how she came to be in Rochester's life, and why she went insane. According to this author (can't remember her name at the moment) the name "Bertha" is bestowed upon her by Edward, it isn't her original name.

Hope that helps.
Edit: BroadwayDoc, that's neat that she's now on Desperate Housewives! Talk about a switch
Q: What is the most weirdest or funniest thing a fan has asked you?
Joe Flanigan, Stargate Atlantis: When a fan asked me for help with his grammar. I'm available.
Updated On: 11/1/05 at 06:32 PM