Whenever a Web browser hits a Web site, it announces itself to the Web site by sending along what's known as a 'User Agent'. Some Web sites look at this User Agent to determine whether or not to let you into their site. For instance, Microsoft's Hotmail only allows Internet Explorer into the site; Mozilla users are not allowed into Hotmail.

However, it is possible to spoof the User Agent that your browser sends to Web sites. In other words, you may be using Mozilla on Linux, but you announce yourself as Internet Explorer on Windows for those few sites who foolishly require a certain browser for entrance.

Changing the User Agent in Mozilla is actually pretty easy, but it requires changing a hidden preference. To set a hidden preference, exit Mozilla (including Quick Launch), go to your profile folder (see http://www.mozilla.org/releases/mozilla1.0/#profiles if you don't know where this is), back up your prefs.js file in case of problems, and create (or edit) the file user.js. Enter this line to simulate IE 5.5 on Windows:

user_pref("general.useragent.override", "Mozilla/4.0 (compatible;MSIE 5.5; Windows 98)");

Restart Mozilla for your changes to take effect.

user.js is a user-created file that lets you keep your custom preferences in a file that Mozilla will not alter (unlike prefs.js). This allows you to keep track of what you have done.

Take great care when tweaking hidden preferences. If you make a mistake that renders Mozilla unusable, delete user.js and restore prefs.js from the backed-up copy.