want to use firefox nightly in a portable apps format on windows?
grab the latest version of firefox portable from here and install to the location of your choice (usb key etc).
get the latest version of the nightly build here (scroll to the bottom and download the appropriate zip…not the exe) and extract it to a temp directory somewhere (not on the usb key). once you’ve extracted it go into the firefox directory (it may be in sub-directory depending on how you extracted the archive) hit ctrl-a to select all the files. do a ctrl-c to copy them and then head over to where you installed firefox portable. now for the firefox directory, which should be in firefoxportable\app\. click into it. delete all the files that are in there then do a ctrl-v to paste all the nightly files, in.
that should be it. start firefox portable in the usual way (FirefoxPortable\FirefoxPortable.exe) and you should be running the new version you chose.
be warned that some of your extensions will no doubt be broken.
and please note that in my defence i have to use windows at work, rather than linux, so i run firefox portable within a truecrypt container through a vpn tunnel. so there.
