You should always say which version of an application you're using when you ask for help. Is there any good reason why you were looking for bookmarks.png, or was it just a guess? :)
In order to find out which files that is installed with a package, use
dpkg -L. I did
dpkg -L firefox | less, and it showed me lots of files, including
But that wasn't it, so I moved upwards. I eventually found
file:///usr/lib/firefox-6.0/omni.jar which seems to provide the default skin. Skinning Firefox is popular, so you'll probably not have any difficulties finding information about that.