Throughout the story, Charles Dickens constantly uses fire as a motif, especially in Chapter 49. What's the symbolic/thematic significance of fire?

An integral part of life in the setting of "Great Expectations," fire is a necessity as food and heat are made.While fire provides a livelihood for Joe on the forge, Joe relaxes in its comfort in the evenings.Yet, while fire provides comfort at times, it is also destructive.  In Chapter 36, before Jaggers advises Pip of his destructive recklessness with his allowance in London, the lawyer gets up and stirs the fire, just as Joe has stirred the fire before he counsels with Pip as a child. 

Fire begins and ends things; it can cleanse an area for new growth, but it can also destroy.  In Chapter 49 both these meanings of fire are present.  When Pip calls upon Miss Havisham she sits brooding into the fire.  Without leaving her fixation upon the fire Miss Havisham asks Pip what she can do for him; finally, after writing instructions to Mr. Jagger, she asks Pip to write "I forgive you" with her tarnished pencil. Pip looks sympathizes 

seeing her punishment in the ruin she was,,,in the vanity..of penitence...of remorse...of unworthiness, and other monstrous vanities that have been curses of this world.

Pip leaves, but has a premonition that he should check on her.  Her back to him, Miss Havisham faces the fire too closely, and her gown ignites. Pip wraps his cloak around her, but she "shrieked and tried to free herself..."  Fire is the cleanser of her vanities that she has long contemplated.