But if Mozilla’s position were really about idealism — tough love for the good of the web in the name of free, open file formats — then in addition to not supporting H.264, they’d drop support for plugins like Flash Player.
Let's assume for the sake of this argument that eliminating NPAPI plugins (and their shapeshifted ActiveX cousins) would be a good thing for the open Web. Would dropping support for NPAPI plugins in Firefox be a good thing for the open Web? Not necessarily. If we dropped support for Flash in desktop Firefox, then (since desktop browsers are so competitive and the switching costs are quite low) a very large number of users would stop using Firefox and start using another browser that does support Flash. So Flash usage would not decrease much. We would certainly not even get close to eliminating the use of Flash on Web pages. On the other hand, there would be significantly less competition in the browser market. Overall, that's not a win for the open Web. Pointless martyrdom can be fun, but it's not good strategy.
The situation with the HTML5 <video> element and H.264 is very different. At the moment, lack of support for H.264 in <video> is not driving users away from Firefox. That is a battle we can still fight and possibly win without destroying our relevance.