Firefox uses a new manager for bookmarks. Bookmarks are no more stored in the bookmarks.html file (in the directory of your Firefox profile), but in a SQLite database (places.sqlite). If the performance is much better, you can not read this file directly (eg in another browser). However, you can automatically export bookmarks to bookmarks.html from Firefox 3. Here's how:
In the address bar of Firefox 3, type: about:config
A warning will appear: Click "I will be careful, I promise!"
In "Filter", type autoexport, then double-click the "browser.bookmarks.autoExportHTML" to pass the value to true.
Firefox now automatically export your bookmarks in the files bookmarks.html
(Just look for the file bookmarks.html in C: Documents and Settings ....)