back button doesn't work first time

I'm running 3.6.16 under Windows 7 on a Dell laptop. For about the last few weeks I've found when Firefox is loaded and I move from the home page to another, the Back button remains greyed out and I can't go back to the home page. This is true no matter what I set as my home page. After this first transition, the Back button works for the second and all subsequent transitions. That is, as soon as I load the third page in a session the Back button is enabled. This was the case in 3.6.15 and 3.6.16. I don't remember whether the problem started exactly when I upgraded from 3.6.14 to 3.6.15 or not.