Cannot shift-click to select text in Firefox

A couple of weeks ago, shift-clicking to select text in Mozilla Firefox stopped working (v1.5.0.2, Thinkpad T42, Windows XP SP2). I can still hold shift down and use the arrow keys and home/end keys, and holding shift-control while using the arrow keys works as well. But throughout my browser (both textareas and normal HTML text), I can no longer click on one spot and then shift-click another to select everything between. I need to do some more testing. Any suggestions?

*UPDATE* It was the AdBlock extension. It disables shift-clicking, and if you double shift-click on something when AdBlock is installed, it (the element you double-clicked on) disappears altogether. Thanks, Neal, for the reminder that Firefox has extensions. :)