Takeaway: If you visit the Firefox website and download the build for Linux, you will receive a 32-bit version of the browser. For someone trying to avoid compatibility libraries, there has to be another way — and there is.
One of the benefits of using Linux is its package-management systems, whatever the flavour of Linux you may use. The one drawback is when there are times that you want or need an update before it can be pushed out of the distribution’s update channels. The application’s inbuilt update mechanism is likely disabled, and the distro maintainers are rightly testing the update — but you need to have that update today.
Typically that would involve going to the application’s website and downloading the update, but for Firefox and Thunderbird users, Mozilla hides away its x86_64 builds on its ftp server; if you go to the regular download page and click on Linux, you will be given the 32-bit build.