Here is how you restore your bookmarks in Firefox 3 by importing a bookmark backup file.
Bookmarks -> Organize Bookmarks -> Import and Backup -> Import HTML and then choose from a HTML file
This is the way for HTML files. Firefox also stores bookmarks in JSON format. Here is how you restore your bookmarks in Firefox 3 using the bookmark backup stored in JSON format.
Bookmarks -> Organize Bookmarks -> Import and Backup -> Restore

Choose from one of the six bookmark backups that Firefox 3 automatically provides for you and you are set.
On a side note I regularly create JSON backups and keep them on my desktop.
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