I'm about to write something sacrilegious with regards to Firefox. I no longer believe it's quite the golden child we were led to believe it was back in January of this year. Right before I cranked up the browser to write this little screed I had to kill the process on my Windows XP SP2 notebook (a Gateway M680 with an Intel Pentium M 2.13 GHz and 1GB of DRAM). Why? Because it locked up yet again viewing a PDF file (an essay written by Bjarne Stroustrup on the future of C++). It occurs when you use the back button to go back to the page from which you launched the PDF viewer in the browser. It's been locking up at least once a day, requiring that I pull up Window's task manager and kill both the Acrobat reader process as well as the Firefox process. And until I kill them both, chew up about 100% of the processor's time.
Reading PDF files isn't the only Firefox killing action. I've also had lockups with Apple QuickTime. And then there's the peculiarity with Java. On the Protege download page you're asked if you want to run the Java-based plugin installer. Every time I answer no, the installer runs anyway, showing up on the page. IE 6 honors that 'no' and the plugin on the webpage does not run. And Firefox, over time, becomes a memory hog. Right now, with five tabs open (Blogger, Slashdot, Ars Technica, the PDF document, and a blank tab) Firefox is eating up 35MB. When it had locked up it had hit well over 100MB. And then there's the performance over time. Open a bunch of tabs (bunch >= 5), minimize the browser, then bring it back to the desktop. You get to see Firefox open painfully slow, and clicking between tabs can force you to wait, sometimes as long as a minute or more while the disk chugs non-stop.
I'm looking forward to IE7. If IE7 answers the key issues of security and standards conformance as well as provides tabs, then Firefox is off this system and every other system I have access to that runs Windows.
Update: Per the comment's advice below, I upgraded from Adobe Reader 6 to Adobe Reader 7. That does indeed appear to fix the lockup problem when moving back from a PDF file in the browser. Thanks to the anonymous poster for the helpful comment. But that still doesn't change my attitude towards Firefox.