Opening 40 tabs in the current Firefox 18.104.22.168 web browser would be the equivalent of going disguised as Hillary Clinton to a meeting of hardcore Barack Obama's supporters; you would be looking for big problems.
Firefox would either freeze, become very slow or display a number of symptoms - like the inability to navigate windows using shortcuts or the absence of right click menus - that would make your browsing experience unbearable.
Firefox 3 Beta 3 is definitely faster, much faster than its predecessor and that's the main reason why you should switch immediately if you don't use any plugins even if it is in Beta mode.
Stephan Shankland from News.com says that it is more stable (haven't used it for much long to vet this) and use memory more efficiently - 40 open tabs are consuming just above 220MB of memory and more importantly, the browser still feels usable.
Firefox 3 Beta 3 also brings in a number of improvements - more than 1300 according to Mozilla's release notes - including a tool to see who owns a website, a more comprehensive URL lookup system, better protection against malicious code and the removal of 350 memory leaks.
However, if you are dependent on your plugins, you might want to wait till the final version before moving and there are some online services that are not compatible with the latest Firefox.
Foxmarks, Fireftp and Alexa's plugins are not recognised and Google Apps may tell you that your browser is not compatible (you just have to append ?browserok=true to the end of the URL; just make sure you press the 'esc' button to grab the URL in time).
You can download the latest beta version here.