You know what I hate?
When I click on a link I almost always want it to open in the current window and tab. If I want to open something in a new tab, I will do so myself using the middle mouse button. This should be my choice. Opening a new window or tab should not be forced on me by the designer.
Which brings me to one of my favourite Firefox extensions: TargetAlert. This excellent add-on gives advanced warning of just what kind of nasty evilness the web designer is about to attempt.
The author: "Provides a visual cue for the destination of a hyperlink by appending a link with an icon that shows what type of file it leads to, or the effects it may have, such as opening a new window or leaving a secure site."
Is there a Firefox extension to add "Open Link in Current Tab" to the right-click menu? That would also be helpful.