Nglayout.debug.disable xul fastload

From MozillaZine Knowledge Base



To improve performance, Mozilla saves a “compiled” version of the XUL and JavaScript to disk periodically so that it does not have to re-parse the data from chrome files on startup. This caching (often referred to as “XUL FastLoad” or just “FastLoad”) significantly decreases the time it takes to do a cold start of a Mozilla application.

The file to which the serialized data is saved differs by platform. On OS X it’s XUL FastLoad File; on Linux it’s xul.mfasl; and on Windows it’s XUL.mfl.

Possible values and their effects


Do not save serialized XUL/JS to disk.


Save serialized versions of parsed XUL/JS to disk for faster startup times. (Default)


  • There is (unsurprisingly) a performance hit associated with setting this preference to true.
  • XUL FastLoad works without the XUL cache on, but FastLoad’s effects will be limited to JavaScript.
  • Conversely, the XUL cache works without FastLoad, but the cache will not be saved between Mozilla sessions.

Previous effects

Until mid-2002, FastLoad (and thus this preference) was only concerned with the serialization of JavaScript. FastLoad began serializing XUL documents after bug 112064 was fixed.

First checked in

2001-07-31 by Brendan Eich

Has an effect in

  • Mozilla Suite (all versions since 0.9.4)
  • Mozilla Firefox (all versions)
  • Mozilla Thunderbird (all versions)
  • SeaMonkey (all versions)

Related bugs

Related preferences

External links