I recently found this cool Firefox trick which coud be a good office practical joke. Just enter this code into your Firefox address bar and hit Enter:

chrome://browser/content/browser.xul

A window of Firefox will open inside Firefox. Reason for this? Who knows..

I tried it in my browser and it worked great, although the the fox suddenly slowed down and became laggy.
Try pasting the url in the address bar of the new FF window. Firefox inside Firefox inside Firefox inside Firefox inside Firefox inside Firefox inside Firefox… see what happends! (;

Disclaimer: I guarantee that this will work with Firefox. But I have no responsibility of any damage that cause Internet Explorer users by Using this with IE to Open Firefox inside IE. (Go on, I know you want to try it)