I've had a problem with Firefox freezing up for a while. I noticed it only seemed to happen when (normal ie. Adchoices) ads on websites came up. The browser will go into "Not responding" I didn't think a lot of it and just knew when it would happen. It would only last a second or 2 and then it would go away without a problem. Well, I got help cleaning out a virus a couple weeks ago and ever since then I've noticed the freezing seems worse. As a second, possibly related, problem I've noticed now video playing seems to freeze up. Youtube as an example. A video will be playing then all the sudden the movement stops but the sound plays as normal. When the movement starts up again the video will fast forward into the right spot and keep going. There's now rhyme or reason as to when it will do it. It's totally random. This video issue DOESN'T go into Not Responding. If I wasn't watching it I really wouldn't even know it was happening.
Note: Firefox is the only browser I use so maybe it's just a Firefox issue...maybe not.