As always, we posted the portable version within a few hours over at PortableApps.com. As we did an extended test of version 4.0 portably following the whole 4.0 beta and RC process, it's turned out to be a nice, stable release. It's great for running from your flash drive, DropBox or just trying out a new firefox install without affecting your local one.
Release Announcement | Mozilla Firefox, Portable Edition 4.0 homepage
I suppose it does depend on your definition of a PC. Most people think Windows, because there is PC (Windows) and Mac. Outside of our geek community, *nix doesn't really exist (some netbooks notwithstanding) in terms of what most people think of. Even on phones, people have no idea that Android is Linux-based.
When outside of our sphere, the world is Windows. If you have your portable apps on a USB drive, you may use them at a local library, a net cafe, a hotel business center, at work or at school. All of those are generally going to be Windows (though a small number of primary and smaller number of secondary schools may be Mac only). Where most of our users wind up using their apps on Wine is on their home computer (or work PC in sysadmin roles or similar), which serves them well as they really only need to configure it, sync some bookmarks, install/update some apps, etc.
In terms of the 'platform being open', we're talking about our platform (menu, backup utility, application updater) as well as our format, installer and launcher/portablizer. All are open source (GPL) and available to all. And they aren't artificially tied to hardware (like Sandisk's now-defunct U3 platform).
It works just fine under Wine. We just recommend setting Wine to Windows 2000 in the version so Firefox doesn't try and do DirectX 9 hardware acceleration (as it does on XP) for most users. I know on my Ubuntu test VM, you wind up with a black window that doesn't redraw if it is set to XP, but it works just peachy set to Windows 2000.
Windows is generally a safe assumption as 94% of the market is using it (then 5% Mac and 1% *nix/BSD).
Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (5) All right, who's the wiseguy who stuck this trigraph stuff in here?