Unfriend Notify is a free extension available for both Chrome and Firefox that alerts you when someone has unfriended you on Facebook. Addictive Tips originally covered it as a Chrome extension, but as it turns out by visiting the extension's Web site, you'll also find a Firefox extension.
Without an extension such as Unfriend Notify, there isn't a straightforward method to discovering who has deleted you as a friend on Facebook. The only two methods available from Facebook directly are to browse through your friends list and try to figure out who is missing, or you can search for someone specifically if you suspect they have deleted you. Neither method is user-friendly, and both can take up a fair amount of your time.
By installing the Unfriend Notify extension you'll find a "Lost Friends" link is placed at the top of your friends list. The link, after someone has unfriended you, will include a number indicating how many people have recently unfriended you.
When Chrome or Firefox is running and the extension discovers someone has unfriended you it will even display an alert, letting you know of the tragic event.
The extension is free, requires no setup other than installation, and starts working instantly. Unfortunately there's no way for the extension to tell you who has unfriended you prior to installing it, but going forward it should work as advertised.