Perhaps that's true for a hobbyist. But any professional software developer should already be familiar with at least one (if not all) of those languages, and should be able to pick up the others with very little work.
Once you know C#, for instance, you'll quickly see that Java is essentially a subset of it these days. Sure, there are differences, but they're quite minor. The same goes for Objective-C, to some extent. The syntax is a bit different, but most of the concepts are similar enough. Even the most commonly-used libraries for each don't actually differ that much in practice.
In fact, many developers with experience in industry will have already had to use multiple languages at the same time throughout their careers. They'll already know how to work on several projects simultaneously, using several languages, and many more libraries.
In fact, I'd suspect that most of these professionals would feel very handicapped if they were forced to use JavaScript for all development. JavaScript has proven time and time again to not be a good language for anything but short scripts. Languages like C++, Java, C#, and Objective-C have core features that make large-scale, distributed software development possible. JavaScript lacks these, and anyone working on a large JavaScript code base will know the pains that arise.
It won't be good for Firefox OS if only the most inept developers are attracted to it. That's not the kind of ecosystem that produces useful applications.