Firefox freezes then stop responding for a few seconds

I don't know why, but my Firefox started to freeze, then not responding message appear for a few seconds (~5 seconds) quite frequent. I think it started to do this for like 2-3 days I think. I tried it in "Safe mode", and it seems it runs without this issue. I'm using only 2 "Add-ons": Adblock Plus 3.3.1 and Popup Blocker Ultimate 9.5. After seeing the problem appears to be gone when it runs in "Safe mode", I tried then to run it again, first with "Adblock Plus 3.3.1" disabled, the problem was back, then I reactivated it, and disabled "Popup Blocker Ultimate 9.5", still I had the same problem, then I tried with both of them disabled, but still the problem persisted. After each try I've restarted Firefox. Seeing that the problem still persist with both add-ons I have disabled, it can't be an add-ons problem. Can you think what else can cause this problem? When this occurs, I've kept an eye on Task manager, and Firefox doesn't seem to use more resources then normal (CPU, Memory, Disk, Network or GPU).