Stop Firefox loading a page based on your clipboard contents on middle mouse click:
In about:config set middlemouse.contentLoadURL to false.
This sounds silly, but it's been driving me nuts. I use the middle button to paste, and if you don't happen to have a text box in focus at the time you can loose whatever you've been entering into the rest of the form.
From:
http://aymanh.com/archives/2006/01/27/firefox-and-middle-click-clipboard-url