I had that exact same problem a while ago but I can't remember what fixed it. I know it happened pre-1.5 and I think it was recently after I modded firefox like so. At the time I thought they were related, I don't know now as the mod is still in effect but the crazy stuff has stopped.
Unless the piece you are referring to is a screw then I can pretty much assure you that it isn't from the plunger. I was just modding an old ed. NF today and the plunger only has two plastic pieces. Good luck.