you can try using 'url lister' addon. paste 40 urls on the text area and click OK.
you can also try open many tabs from mozilla's rss feed if you have it enabled. scroll to the bottom of the folder and select 'open all in tabs'.
if you open too many tabs at once, you will receive a warning.
ram usage will also just significantly with many open tabs.
edit: sorry, i wasn't clear before, this works in mozilla ff. i don't know about other browsers.