After upgrading to Firefox 4 Beta, I found that I was clicking on things multiple times trying to get them to work. For some reason, the default in FF4 is to not have a busy cursor when you click something. Your cursor remains the default arrow with no indication anything is happening. You can restore the FF3-standard (and really Windows-standard) behavior of having a "Busy" hourglass cursor with the following steps:
1) Open a new tab.
2) Browse to the URL "about:config".
3) Click through the warning about voiding your warranty.
4) In the "Filter" box at the top, type "cursor".
5) Double-click on the item "ui.use_activity_cursor". It should turn bold and the value should change from "false" to "true".
6) Close the "about:config" tab.
That's it, no restart required. The hourglass cursor should now be back when you click on a link and the page is loading.