Despite its name, Paste Email, a Firefox extension developed by Chuck Baker, allows you to paste any of up to 16 different short text notes you find yourself typing frequently. It was originally conceived as a quick way to paste your email address but has well grown beyond that.
Paste Email adds an item to text boxes and text areas context menu where you can select one of the predefined text strings for a quick paste. It also adds a handy option to insert the contents of a text file saving you a trip to your favorite text editor, selecting all contents and copying.
A more complete version, Paste Email Plus, by the same developers, allows unlimited text lines and multiline pastes, but I find this capable enough and, unlike Plus, is already compatible with Firefox 3.
My only complaint with this add-on is the name. I would suggest merging the two versions, renaming it, and, specially, rename the context menu option. A Thunderbird would be useful also.
You can get Paste Email (original) from Mozilla Add-ons.