We've carefully ran some numbers before doing this, and this affects a few hundreds to a few thousand people (hard to say, ballpark), and most of those people are on 64bits CPUs, but are using a 32bits Firefox or 32bits userspace.
The comparatively high ratio of 32bits users on Windows is not naively applicable to the Linux Desktop population, that has migrated ages ago.
That's the specific meaning of support that it was my intention to point out. Free software projects usually do not "support" software in the commercial sense, but consider platforms supported when there are enough persons to keep the build alive and up to date with the changing build requirements etc. It was my expectation that Firefox was more like free software project than a commercial product, but perhaps that is not the case?
Commercial products have to care about not spreading their resources thin, but for open source cause and effect are the other way around: The resources available is usually the incoming paramter that decides what is possible to support. Hence my surprise that not enough people are willing to support a platform that has thousands of users and isn't particularly exotic, especially compared to what mainstream distributions like Debian already build.
Debian supports MIPS and SPARC still. Last I checked OpenSSL is kept buildable on OpenVMS. Surely there must be a handful of people out there who cares about good old x86?
If your numbers are correct, there are millions if not tens of millions of Firefox users on 32-bit. If none of them are willing to keep Firefox buildable, there must be something more to it.