I have 5.0 on WIN XP, but the format is the old 3.6 version. How do I get the new format as in 4.0?

I installed 5.0 on my new computer running XP Pro, but the screen format reverts back to the 3.6 version. According to what I see on the Firefox home page the screen format of 5.0 should match what I had in the 4.0 version. Can this be changed somehow? I was just getting used to the 4.0 format and like it better.