I decided to write this for anyone who may be interested in trying out the newest Firefox TraceMonkey 3.1 Beta 2pre nightly builds between now and the time that Mozilla releases the official Beta 2 version. This will also allow you to set up a portable version very quickly after they officially release Beta 2. ^__^
I would like to thank LoneWolf008 for the inspiration to write this up. ^__^
Note: This is also the way that I keep my multiple Portable Firefox installs updated whenever Mozilla releases new 3.0.x versions (exe file method).
Original Comment Thread That Inspired This Write-up
"How To Set Up A Portable Version Of The Newest Firefox 3.1 Beta 1 Quickly" (Started on Tuesday October 14, 2008 by Asian Angel)
Original Comment Thread For This Writeup
"Run The Newest Firefox TraceMonkey Nightly Build As A Portable Version" (Posted on Friday October 24, 2008 by Asian Angel)
I recommend using the stable release of Portable Firefox for this since that is the version used in this write-up. Note: I have not tried this with the Firefox 3.1 Beta available from portableapps.com.
Portable Firefox
♦ Portable Firefox 3.0.3 from Portable Apps/John T. Haller
To get your copy of the newest nightly builds, you can use the following links.
Direct Download Links (FTP):
♦ Newest Firefox TraceMonkey 3.1 Beta 2pre Build Exe File (2008-10-23)
♦ Newest Firefox TraceMonkey 3.1 Beta 2pre Build Zip File (2008-10-23)
Visit The Nightly Builds Page:
♦ Latest Firefox TraceMonkey Nightly Builds
What I am doing is installing the newest nightly build into a portable Firefox installation and overwriting the old version (i.e. 3.0.3). If you have a fresh install of portable Firefox, the default folder name should be "FirefoxPortable". You may have another name for it...here I have changed the name to Portable Firefox.
Here is the location for installing your new nightly build:
Portable Firefox --> App --> Firefox
First, the exe file method...
1. Once you have started the exe file installation, choose "Custom Install". Also at this point make certain that you uncheck the "Use Minefield as my default web browser" box so that this does not interfere with your regularly installed version of Firefox.

2. Select the above mentioned location in the portable folder (based on whether you have it located on your Desktop or in Program Files, or other ^__^). Here I have my portable setup in Program Files.

3. Uncheck the Create icons for Minefield boxes for "On my Desktop", "In my Start Menu Programs folder", and "In my Quick Launch bar".

4. Now you are ready to install your new nightly build.

5. After finishing the install, make certain to uncheck the box for "Launch Minefield now" (leaving it checked will result in the same effect as using the unzipped folder version). You are ready to make your shortcut to the "FirefoxPortable.exe" file in the main portable folder. ^__^

Now, the zip file method...
Note: Running the nightly build directly from the unzipped folder version will access (and possibly interfere with) the profile/extension/theme settings for your regularly installed version of Firefox.
1. Using Windows Explorer or an equivalent app, open to the Firefox folder in your portable install. Note: Here I have my portable temporarily setup on my Desktop.
Portable Firefox --> App --> Firefox
2. Delete the contents of the Firefox folder in your portable installation.

3. Opening a second Windows Explorer or equivalent app window, open the nightly build folder (the unzipped folder name should be all lower case when you unzip it). Select all the contents and drag them to the empty Firefox folder in your portable install.

Now you are ready to move your portable nightly build to the location you want and create a shortcut to the "FirefoxPortable.exe" file in the main portable folder. ^__^
Have fun with the nightly builds! ^__^