I got tired enough of closing a new window in Firefox when I wanted a new tab, I decided to get off my digital duff and do something about it.
This page has a sub-one-minute fix that will get rid of the pesky "Open in New Window" option from your firefox right-click menu. Win.
I showed this to a three guys at work. One was ecstatic. One was interested. One asked if we were really worried about minor stuff like this. Haha.