To improve performance, Mozilla caches chrome XUL documents the first time they load for faster loading later. The effect is that the source XML file is not read and re-parsed each time the chrome in question is displayed. The XUL cache is serialized and saved between Mozilla sessions in the XUL FastLoad file, which saves a “compiled” version of the XUL and JavaScript in a document to disk for faster startup the next time the application runs.
During development of XUL applications, it’s convenient to disable the XUL cache so that changes you make to files on disk take effect the next time the window or dialog is loaded (instead of the next time Mozilla starts).
Possible values and their effects:
True: Do not cache parsed XUL documents and do not save the cache to the XUL FastLoad file on exit; re-read the source files each time the window or dialog needs to be displayed.
False: Cache XUL documents in memory and save the cache to disk on exit to improve performance. (Default)
See also: http://kb.mozillazine.org/Nglayout.debug.disable_xul_cache