From MozillaZine Knowledge Base
You can save a copy of your bookmarks to an HTML file, using the Export feature of the Bookmarks Manager.
Bookmarks exported to an HTML file can be used for backup, in case of future problems with your bookmarks, and can also be used for importing bookmarks into another browser. See Import bookmarks and Restoring bookmarks from backup for more information.
Note: In Firefox 3 and above, you can automatically export your bookmarks at exit to the Firefox profile folder "bookmarks.html" file by setting browser.bookmarks.autoExportHTML to true in about:config but this may delay browser shutdown if you have many bookmarks.  You can set the name and location of that HTML backup via the preference browser.bookmarks.file.