URL Link is a small Firefox and ThunderBird extension that allows you to select a non-URL in a mail/news message or web-page, and open it in a browser window.
For emails, it reconnects links in emails which have been broken across several lines, and also replaces spaces with the URL character code %20 so that you may follow emailed network 'file:' links (which it auto-detects from Windows X: or servdir references).
I frequently receive emails containing long URLs that get broken when either the sender's or my email client inserts a line break somewhere in the middle of it. This allows me to select the URL, right click, and open it in my browser. You can also use it to select any text and have it automatically inserted into your browser's address bar.
Since most people you send email to will not have the URL Link extension, it's best to enclose long URLs in greater-than and less-than signs, e.g., the "<" and ">" characters. (I've tried posting an example with them around a sample URL but Blogger reformats it.) With those characters around the URL, most mail clients will recognize the URL and make it a hotlink, even if a line break gets inserted inside it.