I'm not saying it would be better, but you could shell script your platform's equivalent of:

    start firefox http://example.com
    start firefox http://example.org
    start firefox http://example.net

and that would "open a bunch of specific URLs at once" with a single invocation. Or make a filesystem folder with URL links in it and select many of them and open all at once. Or make a HTML page with some javascript which 'clicks' multiple links at once.

The only thing that does not do is appear in the browser UI itself, and browsers go a long way to try and make sure bookmarks don't either :|