There is but one choice to trust right now in the world of web-browsers for the Windows operating system. Firefox 3.0.11 is the choice for now. The just-released Firefox 3.5 has a whopper of a security hole and I'm waiting for the release of 3.5.1 before I switch.
Truth be told, there are some nits with the new version that would have me holding off anyways. Super-important add-ons Line Marker and NoSquint don't work with the newest version. And Tab Mix Plus pretty well locked up the test machine with the test version of itself in Firefox 3.5 (admittedly, it looks like the old combo of All-in-One Sidebar and Open Tabs Relative now seem to work, making Tab Mix Plus sort of ... unnecessary). But NoSquint is an absolute necessity, and I won't switch until a new version is ready for the latest Firefox.
Now, for those of you using the always insecure Internet Explorer, there are three, count 'em THREE, security potholes that the fine folks at Microsoft have NO IDEA how to fix. Well, that's not true, they suggest you turn off key features of Internet Explorer, lest you end up victimized by 0ne of the many nasty little sites on the internet. And that includes some legit sites you might want to visit, thanks to the insidious frames bug that let's the bad guys overlay invisible sites overtop of the legit ones you think you are looking at (and clicking on). All in all, if you use Internet Explorer, for gawd sakes STOP!
You can temporary patches for the three current stinkeroos for I.E. A good wrap-up article by Michael Horowitz can be found at the ComputerWorld site here. You can directly load the Microsoft msi files to patch your system there.