FireFox extension: How can i modify a file on startup?

I would like to modify a file loaded by Firefox on startup (persdict.dat - this is the personal dictionary file).

What code do i need to use in order to enter an additional word into the file before it gets loaded by开发者_运维百科 FireFox on browser startup?

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See https://developer.mozilla.org/en/Code_snippets/File_I%2f%2fO for sample code used to read/write from files. To get the file itself (I have no idea what persdict.dat is), try to find out if it has a chrome:// URL that can be used to access it. Otherwise, I assume it's in either the Firefox installation directory (C:\Program Files\Mozilla Firefox, or whatever) or the user's profile directory. So use the table under "Getting special files."