Starting yesterday, we began receiving reports of a new memory/cpu usage issue that happens shortly after a normal startup and can spike the CPU and chew up hundreds of MB of RAM. This is apparently happening to people with new profiles or in profiles that have a very outdated list of bad sites for the Phishing Protection feature and the Malware Protection feature.
What's going on is that soon after Firefox is started, Firefox tries to fetch updates to the site forgery and malware lists -- the lists of bad sites that allows Firefox to warn users about suspected Phishing and Attack sites. If the profile has very outdated or no local lists, as is the case for a new Firefox profile, Firefox is trying to bring down these rather large lists in one big chunk rather than slowly in small chunks. This causes Firefox to consume large amounts of CPU and memory and can slow the users machine to a crawl.
I suspect this is the problem that's being reported here, here, and here.
This problem is due to the change in the "SafeBrowsing Protocol" which only affects Firefox 3 Beta 1 and nightly build users. If you're on Firefox 2, this isn't going to affect you.
The work-around for this problem is for us to throttle it on the server side. We're working on that now and I'll post an update as soon as I've got confirmation that it's taken care of.
update: And it should be fixed now -- 1:06 PM PDT (UTC -8)
