Security is definitely a major issue as ScottW pointed out. I do understand BobJam's point on download, but will reiterate and better explain the point, a non-techy person in most cases will not download something when they already have a program to do the same thing. To better describe the user base I am referring to, it is the same person that when they ask you to fix their computer, it takes very long to start up because they didn't really read the options when downloading programs and almost everything they have downloaded starts up with Windows. They also have programs Geeks know to stay away from like WeatherBug. When I was at college/university a lot of people would ask me to look at their computers. I will say though, in concordance with BobJam, a part of this was when Firefox was the secure browser it claimed to be and before IE7 came out. Upon opening IE (as Firefox was not on their computer), I found half the screen covered with toolbars. After a while (sometimes a LONG while), their computer was starting up faster, programs did not start up with Windows, they had Firefox, and I had taught them how to use Ad-Aware and sometimes Spy-Bot: Search and Destroy.
So today with the popularity of Firefox large enough to be on college/university campus's computers, large enough even to set a record in the Guinness book of world records (although to my knowledge no record existed previously), how secure is the browser?
As Geeks we either loath or love Microsoft, whether we use Windows or not, because honestly if we have a "Geek" job, we have to deal with Windows at some point. (Unless you are working for Apple or on a Linux team.) So momentarily let us put away our feelings toward Microsoft as a company, Microsoft as the maker of things such as Vista and Office 2007, and look only at IE6 and IE7.
Why look at IE6 some may say? Some corporations cannot yet switch to IE7 because it will break programs they have. We can assume this will be fixed in an IE7 update but for now, it must stay in the mix.
There are numerous browsers out there but the largest majority uses either IE or Mozilla as the underbelly, so we will look at the main browsers in use today.
IE6 - Windows
One of the lesser secure browsers, but can be integrated with virus scanning software and firewalls. Still has a large user base mainly due to corporations.
IE7 - Windows
Microsoft's most secure browser. Not aesthetically the best according to many people, but not the point. It is more secure than its predecessor, and does take security measures the previous browser did not. Can be integrated with different virus and firewall software.
FF3 - Windows, Linux, Mac
Mozilla's most secure browser. Many aesthetic changes over FF2 that many didn't like, but once again, not the point. It is more secure than it's predecessor. It can be integrated with different virus and firewall software, and has numerous plug-ins to help security.
Safari - Windows, Mac
Horrible Windows browser. As far as a Mac browser, it is better than it's Windows counterpart, but most likely the worst browser on the Mac.
Opera - Windows, Linux, Mac
Quite possibly the best browser available, at least for Windows. Still a smaller user base, but Opera is very secure and very fast.
So what browser to use? IE7, FF3, and Opera are all good browsers. While Opera most likely is the most secure, IE7 and FF3 are not bad enough to push away. The extra features, especially that of FF3 make it worth it. IE7 and FF3 are secure enough to keep one safe if the person stays on safe sites.
As for the big debate of IE7 v FF3. IE6 v FF2 most would agree went to FF2. So did IE7 increase and FF3 decrease ( I say decrease because while more bugs were fixed, the growing user base as BobJam pointed out has made a difference) to make IE now on top? I think at least for now the answer is not so simple because one piece of malware may make it into IE7 and not FF3, while another may make it into FF3 and not IE7.