Tabbed browsing, and not as virus/trojan/etc. friendly.
Upgrading to a new version of Mozzilla won't BREAK every installation in the system by replacing or incompatibly "upgrading" the Win32 "common controls".
Every time I've had to "fix" a horrible emergency where somebody's computer is a useless blue-screen popping ruin, the same theme has emerged:
"I only upgraded to IE XXX".
Both Outlook and Outlook express are only too happy to act on any old scripted junk. Especially if you have that 'preview pane' up. "Oops, I saw that! You wanted to install the latest mail WORM right now! Done! Here, let me send that WORM to everyone in your system-wide accessible address book! I know you don't mind! Oh, and I'll get right on that PING attack you wanted to do!"
Oh, you think your Outlook is secure? So did the people at Valve.
Guess what? It wasn't.
Their whole Halflife 2 source tree was grabbed and put out on the internet!
http://www.planethalflife.com/
So, keep using that Microsoft browser and Mail client. At least the damage probably won't cost you as much as it did Valve.
Our speculation is that these were done via a buffer overflow in Outlook's preview pane. This recorder is apparently a customized version of RemoteAnywhere created to infect Valve (at least it hasn't been seen anywhere else, and isn't detected by normal virus scanning tools).