I'm having the same issue but I want to elaborate a little more on what's happening. I'm not sure what ABP is, but I'm running Firefox 4.0.1 and am experiencing system lock ups as well. I can't definitely say Firefox is the culprit but here's what happens... I'll be doing something that requires the internet (email via MS Outlook, reading email in Mail, browsing the web with Chrome and/or Firefox 4, using Microsoft's Remote Desktop app to manage a Windows machine) and the internet appears to stop working with whatever app I'm using. If I was trying to send an email, Outlook will just sit there reporting that it's sending but it never times out or sends the email. If I try connecting to another computer with Remote Desktop, I cannot. It will just sit there saying it's connecting but never actually does. If I try using Chrome or Firefox to browse to a web page, the browser will say it's loading the page but fails to ever load it. I then have to quit all my apps and attempt to shutdown my system. I can quit most apps normally and only occasionally have to force close them when this happens. However, when I try closing Firefox 4, that's when my system hangs. I've installed 3.6.17 and will see if this continues happening. I suspect it's related to Firefox 4 as it only appears to happen when I have it open (although I don't need to be actively using it) and I never had this issue when I was still on 3.6. I'll update with more details after I do some more testing. I'm also running a Core i7 Macbook Pro with 10.6.7 with 8GB of RAM.