The 'previewHTML" plugin for N++ is very useful as it is, adding other browsers, preferably FF would make it even better though.

While I agree it would be useful, adding a Firefox browser component is very hard to do, if it's at all possible (which I doubt).

You can't embed an installed version of Firefox into other programs. I would have to ship a separate version of the browser component with the plugin, which probably would not match the current version of Firefox. I think that would already severely limit its usefulness.  Also, the embeddable browser component has to be maintained separately; and I get the impression that nobody has done that in recent years.

Even if the browser component is still maintained, it would require a lot of work to incorporate it into the plugin. If I add a browser engine, I would probably try to add Chromium first (Google Chrome's engine), since that's a lot easier to embed.