I don't know of any way of assigning a hotkey like Ctrl-F12 to a bookmark(let) directly in Firefox. However, you have a built-in function in Firefox to assign a key sequence like "abc" to a bookmark.
Combine this with the shortcut for accessing the address bar (Ctrl-L) and you're good to go.
Bring up the properties of the bookmark as described here (http://mozilla.gunnars.net/firefox_bookmarks_tutorial.html#Bookmark_Manager) and enter the sequence in the keyword fiel (e.g. abc, or in this example: Help).
Now in order to get to your bookmarklet, do this:
- Press Control-L
- Enter your sequence
- Press Enter
It is a two-step shortcut but it should do the trick without extensions or adding additional software to your system.