By itself, that's not enough. Firefox doesn't support .mac syncing. It does have an extension which allows for Safari-like bookmark sync across multiple machines, but there's another problem: Safari and Firefox use very different bookmark formats. Firefox embeds its bookmarks in an HTML file, while Safari uses an OSX property list (the same format used for most preferences files nowadays).
All of this said, if you use .mac to symc Safari bookmarks, there's a .mac service which lets you access those bookmarks as a Web page. This may be a good way for you to go, because that page can be accessed by other Web browsers, including Firefox. It should be possible to save that page as a Firefox sidebar, so it wouldn't even be too different from using Firefox's "native" bookmarks.
http://www.mac.com/1/iTour/tour_bookmarks.html