Recently a member over at Go Firefox! had an odd and annoying issue where as each time they opened a new window in Firefox, the Error Console window would pop-up. The Error Console (under Developer Tools on the Firefox Button/Menu) normally just sits quietly in the background and collects errors that have occurred when visiting sites. Useful for troubleshooting a misbehaving website. My first thought was may be they were accidentally hitting the keyboard short-cut for the Error Console when doing the keyboard short-cut for new window. However, while the keys are fairly close together, new windows is CTRL+N and Error Console is CTRL+SHIFT+J.
Next thought was add-ons. Sure enough it seems that certain ‘Mouse Gesture’ add-ons such as All-in-One mouse gestures and Fire Gestures add-ons have been known to cause this behavior. More information about this on the Mozilla Support Forum.