browser.dom.window.dump.file redirects the ouput of
window.dump() calls to a file whose address is specified in this preference if
browser.dom.window.dump.enabled is set to
true. Changes require an application restart.
The value holds the file system path for the file in which the content of the
window.dump() calls gets written, e.g.
C:\dump.txt . It will be created if it doesn't exist yet.
The line breaks have to be in the argument of the dump function, else adjacent dump calls will get logged into the same line.