I was using it this spring (2012) because I had a netbook running Ubuntu 9.04 (9.10 had failed to work properly). But after Google Docs began refusing to let me edit, I finally gave in and upgraded the netbook to 12.04, which has an up-to-date Firefox.

Still, I don't doubt that there are lots of public computers (in libraries, for example) running Firefox 3.6 today. I've seen even older.