If you use the Firefox browser on a PC, you are probably using its "Desktop" application. They also had a version for "Modern" Windows 8.x that could be used from the Start Screen. You probably did not use it because fewer than 1000 people per day did. This is more than four orders of magnitude smaller than the number of users for Desktop's pre-release builds.
Yup, less than one-thousandth.
Jonathan Nightingale, VP of Firefox, stated that Mozilla would not be willing to release the product without committing to its future development and support. There was not enough interest to take on that burden and it was not forecast to have a big uptake in adoption, either.
From what we can see, it's pretty flat.
Paul Thurrott of WinSupersite does not blame Mozilla for killing "Metro" Firefox. He acknowledges that they gave it a shot and did not see enough pre-release interest to warrant a product. He places some of the blame on Microsoft for the limitations it places on browsers (especially on Windows RT). In my opinion, this is just a symptom of the larger problem of Windows post-7. Hopefully, Microsoft can correct these problems and do so in a way that benefits their users (and society as a whole).