I'm don't know a shortcut for the Favorites menu, but you can just bypass it with Firefox "keywords". In the properties for each bookmark is a Keyword field, that's kind of like a shortcut. If you make a bookmark for google.com and put "g" as the keyword, then when you type "g" in the address bar it will take you to google. But keywords are even more versatile than this, and is one on FF's best features IMO.
Combining keywords with the "%s" variable allows you to use FF's address bar as a universal search tool. Go to Google, enter something easily identifiable like ZZZ, hit search, and then bookmark the result. Now go edit the bookmark. Look for ZZZ in the long search string and replace it with "%s" (without the quotes). Give it a keyword (continuing with our example, use "g"). Now when yo type "g Jennifer Aniston", for example, it will automatically take you to the google search results for Jennifer Aniston. In fact FF 1.0 should already have a Quicksearch bookmark folder with Google Quicksearch already created, for reference. It has "google" as the keyword - I just changed it to "g".
You can do the same thing for most any search tool. Some things don't work, but many things do by just searching on some easitly identifiable, and then replacing that string with "%s" and giving it a keyword. I have quicksearches for Google Images ("gi" as my keyword), Google Groups ("gg"), Internet Movie Database ("imdb"), my corporate phone directory, and a bunch of others.
FF rocks.
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