Um, a LOT of best friends in television are like this... Ernie and Bert, Hawkeye and B.J., among others...
Yeah, I guess it does happen a lot (though I haven't seen Sesame Street very often, so I can't really such how they would do that on a show like Sesame Street). But it's just that, with Muffy and Francine, it's gotten to the point where it's hard to even determine whether they're friends or enemies. Most similar best friend pairings usually have the two either casually teasing each other or reconciling at the end of an episode. But between these two, like I said, it's almost hard to tell.
Though I guess one could interpret the "hate" part of this relationship as casual teasing.
One of the most similar parallels in another show that sticks out to me is Cyborg and Beast Boy from
Teen Titans. They're known to be best friends (in fact, Beast Boy even said so in "
Stranded") but at other times, they are arguing in a rather unfriendly manner. Though I can't shake off the feeling that they are different from Muffy and Francine...mostly because they both are part of a larger team, I guess.