With Firefox 1.5 a new backup feature was introduced to protect your bookmarks from accidental deletion. Firefox stores daily backups in the bookmarkbackups folder in your profile folder (in Windows, typically in
C:\Documents and Settings\\Application Data\Mozilla\Firefox\Profiles\.default\, check here for profile location in other operating systems and versions). Backups are stored for the last five days and are called using the pattern bookmarks-YYYY-MM-DD.html.

So the next time you accidentally delete some of your bookmarks, look here for the most relevant backup, copy it to your profile folder and rename it droppping the date part of the name, overwriting the bookmarks.html file.

For more restore options check the Mozilla Knowledge Base.