At the top of your Firefox browser, click on the Bookmarks Icon (it looks like a star), then click on Manage Bookmarks, then at the very top of that screen, click on Import and Backup, then choose from the array of available options to incorporate your bookmarks file. Hope this helps.

One further note:

If the file is a .json file created using the Backup feature, or a .jsonlz4 file that Firefox created automatically:

Use the Restore sub-menu and click Choose File to point Firefox to your backup. See: Restore bookmarks from backup or move them to another computer

If the file is a .htm or .html file created using the Export feature:

Use the Import command. See: Import Bookmarks from an HTML file.