Linkify is a command for Ubiquity - a neat Firefox extension, which allows you to do all kinds of neat things within your browser, by telling it what to do rather than where to go.
Linkify allows you to quickly and easily turn plain text URLs into clickable links - simply select the URL, call up Ubiquity, type "linkify" and hit enter. If you'd like to specify the link text, type "linkify as [your link text here]" - for example, "linkify as foo bar" - and hit enter.