Customers became so fed up with the security vulnerabilities in Internet Explorer 6 that many pundits rallied to leave the browser for an alternative such as Mozilla Firefox. Yet even beyond the feature-centric decision points—such as which browser implements Tabbed browsing better, provides better support of RSS feeds, or has a better UI—your decision won't be entirely security driven. Many companies simply won't be able to switch from the IE platform if they rely on Web applications that leverage features available only in IE. For example, across the board, Microsoft SharePoint simply works best with IE, and some Web parts work only in IE. Also, many people see IE-only technologies such as VBScript and ActiveX as security risks, but many companies include these technologies in their Web applications. Some companies simply won't be able to switch from IE to another browser cost effectively because of the dramatic changes that could be required by the applications they use.
If you don't use IE-only applications, your decision might be influenced by timing, feature set, and application management. If you can't wait, your only choice is Firefox because IE 7 won't be released for a few more months—and the best version, which will be included in Vista, won't be released until the second half of 2006. Remember, however that although Firefox is a great browser, it too has security vulnerabilities that you must manage and patch, which brings me to my next recommendation. Microsoft includes IE patches with the rest of the Windows patches, so if you use any patch-management software or service, you'll be assured that your browser patches will be included. Firefox includes a built-in updater that can automatically fetch and install updates, but it's not centrally managed. Also, IE integrates with Group Policy Objects, so you can manage browser settings for larger deployments and centralized configuration.
IE 7 will include security features not available in any other browser, and I believe the Microsoft team will do everything it can to ensure that it doesn't face the notoriety of earlier versions. My recommendation is to download the beta and begin testing IE 7 as soon as possible so that you'll already be familiar with it when it's released. It's shaping up to be a huge improvement over earlier versions and will leapfrog Firefox in terms of security and management features.