Eh, I'm not so sure. If malware can inject itself like that, it can already have its way with your data and run whatever code it wants. Would it really need to escalate to root?
I hate having to press Ctrl+Alt+Del to log in. It's an awkward, two-handed chord, and Windows uses it as a crutch because of its inferior security model. Sure, if Linux had such a system from the kernel up through X, it'd be a tiny bit more secure--but with the fundamentally more secure model, and by using trusted binary repositories, I don't think it's necessary. Besides, what are you going to do, press Ctrl+Alt+Del every time you have to type your password? Ugh!
BTW, SELinux on Ubuntu works quite well with Firefox. I can't vouch for how well it stops exploits, but it's there, and is kept up-to-date by Ubuntu.