Why Firefox Rocks: Great Firefox Tricks, Part IV - page 2
The Plague of Ridiculously Long URLs
Plain text URLs like that are fairly straightforward. You tend to see them in places like blog comment threads or social networking sites. When you see a split URL in email, it's a little different. Most email programs try to detect web URLs and make them clickable -- but there's no way for an email program to know for sure whether a URL has been split over multiple lines, so usually it highlights just the first line. So you might see something like this:
Now the situation is worse: it looks like there's a link you can click on, but if you click you go to the wrong place. That's why broken long URLs cause so much mass mailing list confusion.
But fortunately, those are almost as easy in Firefox. They're just a little harder to select, because if you click at the beginning, Firefox or your mailer may think you're trying to drag the link to some other window. Instead, start from the end of the last line and drag backward and upward to the beginning. Then paste into Firefox just as before.