There's another possibility. If you've ever installed Firefox/Firebird/Phoenix before, then you've got an old profile on your computer. Since Firefox is still in beta, they've felt no need to keep profiles compatible from version to version. This can cause the symptoms you've observed.
How to fix:
1) Copy the "bookmarks.html" file from your current profile into somewhere safe - your documents folder or desktop for example. This can be in any number of locations; some possibilities are ~/Library/Phoenix, ~/Library/Firebird, ~/Library/Firefox, ~/Library/Application Support/Firefox.
2) Delete all existing profile directories for Firefox/Firebird/Phoenix.
3) Run Firefox, then once it's done launching quit the program. Copy your backed-up bookmarks.html file into your newly-created profile directory - overwriting the existing bare-bones bookmarks.html file that was created for you.