firefox portable is a great creation of portableapps.com, however unless you are content with being on the public release channel of firefox it doesn't cut the gravy. This guide will tell you how to get on (at the time of writing, which was shortly after 4.0 hit public release) the 7.0, 6.0, and 5.0 channels.
stable, 4.0- head over to portableapps.com to get it. under applications, run the.paf.exe and open it up
beta, 5.0- run the .paf.exe from this page http://portableapps.com/apps/internet/firefox_portable/test
aurora, 50.a2- get beta, then open up the firefox menu-->help-->about firefox-->change-->select aurora
nightly, nightly, 7.0a2- get any of the above, go to http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest- trunk/ and download the latest .zip for your platform example is firefox- 7.0a1 .en-US.win32.zip. extract it. go to the extract folder "firefox" , delete EVERYTHING then paste the files in from the package from http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/. run firefoxportable.exe. the first run page should appear. open the nightly(firefox) menu, help, about nightly. check that it says nightly, not firefox and hit check for updates