InformationWeek has a long article covering the quality issues with the Firefox 1.5 release. Basically, lots of users have memory leak problems, hangs and display problems. Not to mention all the extension incompatibilities. I've seen a lot of these problems too.
Do NOT upgrade to 1.5 unless you have some very good reason to do so. Wait for the patch releases to come (in February).
However, I worry when I read quotes like this from the Mozilla execs:
"We have more than 10 million downloads of Firefox 1.5, and the overwhelming feedback we've received ... has been positive," engineering VP Mike Schroepfer says. "We have heard some reports [of high memory use]..."
That's PR-speak. I hope that they are much more serious behind the scenes. Don't think that Mozilla doesn't have the resources to handle the QA problem much better than they are. Mozilla has more cash lying around than they know what to do with.
Even though it's kept secret, it's likely that Mozilla gets more than $30M / year from Google alone. They also have deals with other big companies like Nokia.
That could buy a lot of QA. And Firefox needs it.