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.
Estimated installation time: 2-3 minutes (1 file upload, 1 template edit, 0 product files)