This article explains how to import an HTML bookmarks file formatted in the Netscape Bookmarks format.
Before proceeding, you should make a backup of your current bookmarks. You can do this by opening the Bookmarks Library window (explained below), then selecting Import and Backup and choosing Backup…. This will create a Firefox bookmark backup file in JSON format.
Click the Bookmarks button and select Show All Bookmarks to open the Library window.
The bookmarks in the selected HTML file will be added to your Firefox bookmarks within the Bookmarks Menu directory.