Running Firefox from a tarball is a very specific use case for a small subset of highly technical users. Even if you figure out how to do it, the lack of system integration gives a very poor impression of the way things work on Linux. Even the vast majority of technical users don't want to run Firefox this way. Yet that's still what the website offers you.
We should be able to recommend at the very least Ubuntu to average users. And people do that, sometimes quite successfully. But with the attitude that all Linux users can save themselves - "we don't need to think about the UX" - we can't really keep doing that.