This is easy. You can do it in only a few steps.
Portable Applications (or PA for short) already has portable Firefox and portable Firefox Beta. A few other websites also create their own packages, including Nightly builds, to be installed in your portable app folder. But, PA does not support Firefox Nightly build. So you have to do it yourself.
There is a page on the PA forums that tells you what to do in order to install your own portable Firefox Nightly build. and here is what it says:
- Go to http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/ and download the latest trunk in zip version
- Use 7-Zip Portable to extract the contents of the zip file
- Copy the contents from the folder firefox to [Firefox Portable Install Folder]\App\firefox
Just remember that PortableApps.com doesn’t support nightly builds, and there’s no guarantee that it won’t let any traces behind after you close it.