By PC, I guess you mean Intel. Still, a magnitude more than what IE runs on.
FireFox runs on all PC platforms. That's not the same as all systems.
As far as accountability, a very good point was made earlier in the MS is not accountable for its goofs now while open source runs to fix their problems.
So let's see you get on any MS forum and tell them to fix CSS on IE7 at any time of day and see how quick it gets done and if you get ANY response, ever!
Will get it done? Naw. Maybe. If you're lucky. And ask nicely. On the right forum. At the right time of day.
At least it's closer than IE which can barely supply a recognizeable image. But let's get real with something we use. How about the DOM. Can't pass anything since 1998. How about Css2.1 while other browsers are working on CSS3. How about the DoS virus already out on IE7? How long will that take to fix? How about XHTML. Doesn't work in IE7 at all and no one knows when! Talk about bugs that are more than ONE year old. Let's start with the last century with IE!
Which is why FireFox still doesn't pass the Acid2 test, still has several vulnerabilities, and still has dozens of bugs that are more than a year old.
If they make the same decisions and they are both smart people, what is the MS problem with IE? Why can't they make it work? Why has it had NO updates to CSS/HTML/DOM/anything but security holes since 2001 in IE6 and 1998 for all IE? Why is Firefox, just barely over a year old, light years ahead when it was designed by some college student in his spare time? Did this kid outrun all of the smart MS people on their own? How did this happen? I guess this kid just wasn't accountable.
dozens of studies have shown that closed source and open source teams typically make exactly the same decisions. Because generally they are both run by very smart people (at the level we're talking about anyways).