There are many reasons you may want to hide the Bookmarks menu in Firefox. Usually because you have some add-ons that give you similar functionality that you want to use. What ever the reason here is how you hide it.
Open the file userChrome.css which is located at: C:\Documents and Settings\<your username>\Application Data\Mozilla\Firefox\Profiles\<some code>.default\chrome\userChrome.css
In Firefox 2.x add the following to this file
In Firefox 3.x add the following to this file