Rated 5 out of 5 stars

Very good idea. Any plans of adding it to the original Firefox (let's say, the 3.6 beta)?

This review is for a previous version of the add-on (0.17.2). 

Built-in support for MAFF and MHTML in Firefox has been suggested, but is not likely to be done soon

At present, there are no plans to add the MAF extension as it is to the core. It has been suggested, however, that support could be added for the two web archive formats, MHTML (https://bugzilla.mozilla.org/show_bug.cgi?id=40873) and MAFF (https://bugzilla.mozilla.org/show_bug.cgi?id=381413). This effort is not likely to start any time soon, as it does not involve a simple inclusion of the extension, but a far more intensive work.