What devices and hardware are Firefox users using? - reliable data available from Mozilla

It's always been difficult getting reliable data on what percentage of PC users use a particular operating system.
Why might it matter? Well, if 10% of Firefox users are on Windows XP - as is currently reported - you can make much more reliable assumptions about things like what font file formats the market, in general, might require or, as another example, what rendering engines are likely or not likely to be interpreting your fonts - GDI or DirectWrite to name two. 
Hat tip to Anselm Hannemann, curator of the Web Development Reading List for the following info.

• Firefox has introduced Telemetry a while ago to its browser and now shares some details on what devices and hardware Firefox users use. In September 2016, for example, 10% still used Windows XP while only 7% used MacOS and 77% of the users still have Flash installed. The most common screen resolutions are 1366x768px and 1920x1080px. There are many really interesting statistics in there, and we’ll have to see how this develops over the next few years. But for us web developers, this also highlights that we shouldn’t assume that people use QuadCore CPU, 8GB RAM machines but have “lower-end” devices instead. So be aware of this before you create fancy CPU/memory-consuming web applications that a user will not have fun with.

You might want to bookmark the page for future reference. 
https://metrics.mozilla.com/firefox-hardware-report/#goto-os-and-architecture