That I can write this review in emacs rather than the little textarea on mozilla.org is wonderful. I use It's All Text! almost daily, mostly with the TWiki on our intranet. I have created an emacs major mode associated with a suffix I added to the It's All Text! list, so I can edit TWiki markup very easily.
All is not quite perfect, however. Another has asked for a different command line per suffix so that we needn't create scripts to manage the demultiplexing. While I can handle that in emacs, I can see the value of different command lines for each file type. That idea doesn't go far enough, however. IAT always uses the most recent suffix selected for all pages, but there is a better way: manage a default, but allow us to associate different suffixes with web sites and pages.
IOW, provide a list of suffix/command pairs and allow users to associate a suffix with a web site or even an URL prefix. By the latter I mean that IAT could match the current page's URL against the list of custom URL prefixes. The longest match would provide the suffix to use and, thereby, the command to run. Thus, entries for "https://addons.mozilla.org/en-US/firefox/reviews" and for "https://addons.mozilla.org" would mean that the former would match when adding reviews for Firefox Add-ons while the latter would match when editing any other form at mozilla.org. With different suffixes associated with each prefix, IAT can run different commands for the two contexts. If no custom prefix matches, then IAT should fall back on a default suffix which the user should be able to specify.