One of my favorite features in Firefox is the Error Console in the Tools menu. It's a Developer feature inherited from Netscape that has been enhanced over the years. Whenever a page has a Javascript error, the precise error message with line number in the source code shows up in the Error Console. When you click on the error message, you can jump to the exact line in the source.
IE 8 (Beta 2) introduces a similar Console under the Script tab in Developer Tools. To invoke this hit F12 (for Developer Tools), CTRL + 3 (to jump to the Script tab which is third in the tab list) and then CTRL + ALT + I to go to the Console window.
You can also choose to see only Script Errors.
Also see:
Developer Tools: Keyboard Shortcuts