Added support for the firefox.preferences system property.

The firefox.preferences system property lets you set arbitrary preferences in the Firefox profile. You pass them in as a semicolon-separated list of values, e.g.
-Dfirefox.preferences="browser.download.folderList=2;browser.download.manager.showWhenStarting=false;browser.download.dir=c:\downloads"

Integer and boolean values will be converted to the corresponding types in the Firefox preferences; all other values will be treated as Strings. You can set a boolean value to true by simply specifying the property name, e.g.
-Dfirefox.preferences=app.update.silent