Mozilla.org says it fixed a problem with Firefox 3 Beta 1 that caused it to spike CPU usage and eat hundreds of megabytes of RAM. The fix involved changing the configuration of a server that Firefox communicates with, so you don't have to download a new version to take advantage of it.
The bug affects Firefox installations with very outdated lists of bad sites for the phishing protection and malware protection features of the browser, according to a report on Mozillazine.. It also affects installations that have no list of bad sites at all. "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," according to the blog author, Asa Dotzler, community coordinator of Firefox marketing projects.
Dotzler said he thinks that was the cause of a problem I encountered running Firefox 3 Beta 1 on a five-year-old Compaq laptop, where Firefox 3 B1 ran so slowly as to be useless, even though Firefox 2 runs fine. I haven't tested it out on that machine to see if the problem is, indeed fixed. I've been using Firefox 3 B1 as my production browser since I installed it Wednesday, the day it became available, and I've found it to be very stable, even though others have reported problems.