This simple client-side add-on saves messages on-the-fly so that if you close your browser on accident or your computer crashes, your post will still be in the editor when you navigate back to where you were.
The way it works is easy: as you type a message, it gets stored in your browser's localStorage. Once you submit it, the copy gets erased. If you close your browser and then re-open a page, the saved content will automatically be re-inserted into the editor.
Stored messages are keyed by URL, meaning you can have more than one saved draft at a time, but anything after the #symbol is ignored.
Works for replies, quick replies, new threads, and PMs.
Supports IE8+ and all Gecko browsers, uses jQuery.