I experienced the same thing a while back, and discussion at the time
eventually traced it to javascript code on the page(s). Go to one of the
offending pages and watch the whirlygig that tells you FF is working to
load a page. You will likely see that the page loads, the whirlygig
stops, then starts again and loads the page again. Look at History,
you'll see the page loaded twice.
I recall that
washingtonpost.com was one of the offending sites for me
(but hasn't done so in a while).
Or, it could be something else entirely, and I could be badly out to
lunch. It has happened before.
Bill