The preference nglayout.debug.disable_xul_fastload controls whether or not XUL fastload is used.

Never set nglayout.debug.disable_xul_fastload to true in a production environment; it exists solely to aid in debugging. In particular, add-ons should never change this preference.
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Values

false (default)
XUL fastload is used.
true
XUL fastload is disabled.