... There are only 3,000 Linux Firefox users out there ...
I find this hard to believe. Firefox is installed by default on many of the most popular Linux distributions (such as Ubuntu, Fedora, etc.). When you consider the millions of Linux users using these distros (yes, I can back that up with reliable sources - can you say the same?), it's not hard to imagine that the number of Firefox users on Linux extends well over a million (maybe millions).
Bear in mind that Firefox is usually installed via a package manager, so the number of Linux downloads (if any) from the Firefox website are not to be relied upon as a true indication of the amount of Linux users using Firefox. In addition, there are also many of us who prefer to compile from source, as we are then free to modify and tinker, or just study out of academic curiosity.
Besides, although I'm not familiar with the Firefox codebase, I'm pretty sure that there is no "Windows version", and it's just recompiled for each operating system. Development "of FF for Linux" is, I think, not an issue.
Just my 2c.