This is what finally worked for me with Firefox 3b5:
Using the Bookmark-Manager in Firefox (Command+Shift+B), I exported
all of my current bookmarks into a new "bookmarks.html" and placed it
into the folder "~/User/*mymacosprofile*/Library/
Application Support/Firefox/Profiles/*myfirefoxprofile*.default/"
overwriting the old one.
Now all bookmarks show up with the normal Firefox plugin for
Quicksilver, adding a custom catalog entry isn't necessary.
Of course, this "bookmarks.html" is not updated, because as Zax said
they are stored in the "places.sqlite" file. But this is better than
having nothing at all, right?
