使用Firefox的插件创建跨网域ajax请求

我有点困惑，如何你可以或不能在Firefox的addons中做跨域的ajax请求。像LastPass和Xmarks这样的插件提示你可以这样做，但是当我尝试google时，大家似乎都说你不能，除非用户在Firefox的设置中设置了首选项。如果不能使用跨域，那么Xmarks，LastPass和其他插件如何处理与其各自服务器的通信？

I'm a bit confused about how you can or cannot make crossdomain ajax requests in addons for Firefox. Addons such as LastPass and Xmarks suggest you can do it, however when I try to google how you do it, everyone seem to say you cannot, unless the user has set a preference in the settings of Firefox. If crossdomain isn't possible, then how do Xmarks, LastPass and other addons handle the communication with theirs respective servers?