I noticed that too when I looked at my start menu items and it showed two different Firefoxes.
Your portable Firefox will have all it's files in one folder and have it's own profile. It will be a totally separate install from your normal Firefox. You can't run them at the same time though.
if you want to actually test the new firefox don't download the beta builds .. get the nightlies .. its called Minefield .. its just a code name for firefox ..
http://forums.mozillazine.org/viewtopic.php?t=630644
these are the most up to date builds and have automatic updates everyday .. the beta versions are for developers ..
edit: you can install Minefield and still use fx2 ..