Give me one site whose videos you can't watch on Firefox for Windows or Mac currently.
The only real contender is Netflix but only once the Silverlight plugins reaches EOL. At that point Netflix will have to choose if they want to risk loosing 16% (est.) of their Firefox customers. Of course if Mozilla decides to implement DRM right now it won't be an issue for them.
That's only if in the toss up between losing Netflix and changing browser, they pick changing browser every single time. Which they won't.
One thing you might not have thought about, is that at the moment, we have Flash, Java, Silverlight, etc which all periodically are found to have mahoosive security flaws - because they can do general purpose things. If Firefox moves to supporting a plugin which is used for exactly the same thing, but that can only do decryption, then suddenly the browser becomes much more secure. If anything, it's an advantage.
What is the proportion of Firefox users using Netflix. Most of Firefox users are in other countries than the US which typically don't have access to Netflix anyways. As soon as DRM becomes accepted by all major browser vendors nothing prevents other site from adopting the same thing which then spreads the usage of DRM. I don't think this is desirable.
For DRM to work it needs to have direct access to the hardware. Otherwise it's too easy to run the plugin in isolation and capture the output. So while the API surface is smaller it's still problematic because DRM bypasses all the sandboxes. Issues like cross-platform support still aren't resolved because DRM providers don't typically provide linux runtimes. I really don't see where this is an advantage versus no DRM.
The only real contender is Netflix but only once the Silverlight plugins reaches EOL. At that point Netflix will have to choose if they want to risk loosing 16% (est.) of their Firefox customers. Of course if Mozilla decides to implement DRM right now it won't be an issue for them.