Triple-clicking (as per Sir.Prize's first method) doesn't work for me with Firefox. However Sir.Prize's second paragraph describes my method. (Except that, being left-handed, I find it more natural to 'click and drag' from the end of the selected text to the beginning, in order to highlight it).
Chris
PS: If you haven't already installed the Adblock Plus add-on, I strongly recommend it. It speeds up the download times of advert-heavy sites like AB enormously:
http://tinyurl.com/2cd87q