Making Crossdomain Ajax Requests With Addons For Firefox

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

Solution 1:

You most definitely can make cross-domain XMLHttpRequests in Firefox add-ons.

There's a reference to it here: https://developer.mozilla.org/en/Extension_Frequently_Asked_Questions#I_cannot_initiate_an_XMLHttpRequest_from_my_extension