one JS_MaybeGC after reading in ALL the default prefs, and do a JS_MaybeGC after reading in any single user prefs file (i.e. prefs.js) sr=brendan@mozilla.org, r=jst@netscape.com git-svn-id: svn://10.0.0.236/trunk@84284 18797224-902f-48f8-a5cc-f745e15eee43