Firefox and Flock are built on the same basic code. Part of that code doesn't allow for the migration of favorites from other Mozilla browsers while the program is running. That stinks, we know. We would recommend that if you have favorites you would like to import from both Firefox and another browser that you import the Firefox Favorites in the initial setup, and then import your other browser's favorites later.
There is a workaround to this if you have already finished the first-run importer. Since Flock is built to manage favorites from social bookmarking sites like del.icio.us and Shadows, you can upload the favorites to the service of your choice from Firefox. Then, run Flock and configure favorites sharing to use the account you just uploaded your Firefox bookmarks to. Voila! Now you have access to your Firefox favorites in Flock!
