what operating system are you using?
If it's something windows based, search in
C:\Documents and Settings\<User Name>\Application Data\Mozilla\Firefox\Profiles\<your profile>
You might find a file that contains your bookmarks (the current bookmarks file is usually named "bookmarks.html". There is also a backup of this named "bookmarks.bak") that's dated before you lost all your bookmarks. If so; delete the current bookmark file, rename this older one as the one you just deleted. Making sure of course that Firefox is not running.
If you don't find such a file; when the only other way you can recover it that I know of is if you are using something like Norton Utility which create a trash can you can recovered file from. If so then search for a bookmarks file that's dated before you lost your bookmarks. (named as as either file I mentioned above). If so recover it and you should be OK.
If not then you're probably out of luck. I recommend that you download this program here
http://mozbackup.jasnapaka.com/and backup your Firefox (and Thunderbird if you use it) on a weekly basis (or as often as you see fit) in case this happens to you again.