The most important fires in Jane Eyre are Bertha’s two acts of arson: the first at the end of Book I (Chapter 15), when Bertha sets fire to Rochester’s bedclothes, and the second at the end of Book III (Chapter 10), when Jane learns that Bertha managed to burn down Thornfield by setting fire to what was once Jane’s bedroom. She’s a real pyromaniac, that Bertha. Anyway, let’s repeat that first one in case you missed it: Bertha sets fire to Rochester’s bedclothes. Since we know that one of the main problems in Bertha and Rochester’s marriage was that, even though she was beautiful and attractive – Rochester tells us that she was "in the style of Blanche Ingram" – she had a rapacious sexual appetite. She was "unchaste" and "dragged" Rochester through "hideous and degrading agonies." To translate that out of the polite nineteenth-century jargon, she cheated on him a bunch and everybody knew about it. The sexual desire Rochester originally felt for Bertha, the sexual attention she drew from a crowd of admiring men, and her affairs – all of this sets his bed on fire, nudge, nudge. Jane extinguishes the literal flames only to kindle new ones of a metaphorical kind!
When, much later, Bertha takes her pyro tendencies to Jane’s bedroom, she seems to be objecting directly to Jane’s own sexual interest in Rochester – but also, perhaps, transferring her highly-sexed nature to Jane. Don’t forget that Jane calls herself "fire" when she’s talking to St. John Rivers – one more thing she and Bertha actually have in common. Bertha’s arson also symbolizes her using the power of sexuality to destroy Rochester’s home; Thornfield actually burning down is a real-world parallel to its metaphorical burning – Jane’s abandonment of Rochester after his desire for her caused him to attempt bigamy.