Firefox config: How to set settings that are not visible?

I would like to set a setting (nglayout.debug.disable_xul_cache)开发者_如何学运维 to true. But when i enter "about:config" in my FireFox and search for this setting i cannot find it.

How can i set this setting?

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Right click on the screen in about:config, select New -> Boolean and enter your setting name (nglayout.debug.disable_xul_cache). Then select true, click Ok and you're done.