By default, Firefox includes a copy of the XUL.mfl (Windows), XUL.mfasl (Unix), or XUL FastLoad File (Macintosh) file. This is the Mozilla FastLoad cache that resides in the profile area. You can set the following preference to prevent it from being used:-

nglayout.debug.disable_xul_fastload     /* default = false, set to true */

When FastLoad is at work, Firefox reads files directly from the chrome only if it needs to, which saves disk access time.

The general-purpose web cache is used to cache XUL files whether FastLoad is at work or not. To turn that other caching off as well, either disable caching generally as in the previous example, or else set this preference:

nglayout.debug.disable_xul_cache    /* default = false, set to true */

A further hack described by some pundits is to delete the XUL.mfl file while Firefox is shut down and replace it with a directory of the same name. That makes Firefox think the FastLoad cache is corrupt or missing, and XUL files in the chrome are thus always reloaded from cache or directly. This kind of trickery is necessary only if preferences can’t be set for some obscure reason.