Is GitHub working for you right now or is it an OS X 10.7 Firefox browser issue?
It is an issue with OS X 10.7 in the sense that this operating system is very old and (most importantly) unsupported.

As it is unsupported, Mozilla have decided not to support recent versions of Firefox on this OS. This is similar to decisions taken by Google and Apple and other browser vendors.

As GitHub requires newer Firefox, and newer Firefox requires newer OS X - you're basically stuck. You should update OS X to continue - or if you cannot do that, then upgrade the hardware or use a different operating system.

You never want to be using an operating system that went out of support years ago (in this case October 2014). The security risk is very high, and logging in to usually sensitive sites such as GitHub should not be done from an unsupported environment.

In regards to your edit, GitHub works fine if you use it with a modern system. It is NOT because GitHub’s join page is broken or down.