Ok, so I've had this problem for a while now and it isn't Firefox cause I tried in Exporer also.

There are several sites that don't load up right when I visit them but the most notable/memorable/widely-known one is facebook. For example, when I "poke" someone back, and there is that second pop-up thing, I click ok but the only way I can get it to go away is by refreshing the page, and often times I have to reload the page anyways cause nothing shows up. Also, if there is something that pops up and you can click no or whatever and it's supposed to go away, if it goes away I can't click on the page and have to reload it, often times causing quite a frustrating circle. Is there a plug in I'm missing? or something like that? Or could it be my anti-virus?

XP Media Center edition.

I guess no one knows?

Sorry, I really have never heard of that before.

Since you say it isn't a problem in IE, I would try uninstalling then reinstalling firefox. Yea, it is a pain but it may fix your problem.

EDIT: ignore me, I missread your post.

happened to me once , the exact same problem and did everything but nothing worked till I called my service provider, there was a problem but got fixed within an hour.
before calling them try this:
go to network connections > right click on the connection u r using > choose repair
this may fix it.