Mike Hoye's Navigational Instruments (via) is about a bunch of underknown or underappreciated features in Firefox. I learned some really useful tricks from the article, including this one:

• Holding down Alt while selecting text allows you to select text within a link without triggering the link

(In fact I used the tip to copy the title of the article from the article, because in the article the title is a link to itself.)

You may already know some of these tricks, as I did, and not care about others (I don't make much use of the URL bar, now called the 'Quantumbar'), but there's likely valuable stuff here for every Firefox user.