Using a beta edition web browser is something most people don’t want to do, especially when it comes to Firefox. Why? Because there’s a risk it may break one or more of the add-ons you have installed in the browser. And it’s a pain to do all that install/uninstall crap because it’s not just for the browser but all your add-ons as well. This can prove to be irritating in short order.
Is there a way to use the new Firefox without actually installing it on your hard drive? Of course there is. You can install it to your USB stick where it will stay self-contained.
Firefox 3.5 Beta 4 does have a PortableApps version, and said honestly this is the absolute best way to try out the browser without messing with your existing 3.0.10 that’s on your hard drive now. Not only can you use it but also test your add-ons to see which will work or bust.
For those that haven’t used PortableApps before, here’s how to use it:
First, find a USB stick you’ve got lying around somewhere. Even if it’s an ancient 128MB stick, that’s fine. Erase/format the stick to free up the space.
Second, after installation, make note of the drive letter assigned to the stick. For most people this is drive E or F.
Third, download PortableApps and install it. When it asks where to install to, use the drive letter assigned to the USB stick.
Note: I suggest downloading the "Platform Only" version of PortableApps, especially if you have a stick with a minimal amount of space on it.
Fourth, download FF 3.5 beta 4 from here. Run the installer (the installation directory should point to the PortableApps directory on your USB stick).
Run the browser from PortableApps afterward, and that’s all there is to it.
Looks like this when finished:
On launch:
Confirmation it is 3.5 beta 4:
To those wondering, no, you cannot run FF 3.0.10 and 3.5 at the same time. That was the first thing I tried. 
Regardless of that, it was cool I can run this browser without having to actually install it to the hard drive.
Once launched it’s quite speedy even though it’s running off USB.
If you want to try out add-ons, go right ahead; they will be installed to the stick directly.
To those with slower PCs and/or (gasp) USB 1.1, there is the option to not store session information when you first run FF. Otherwise if you’ve got USB 2.0 and a relatively decent PC speed-wise, you can have it store sessions and it runs just fine.
A) It doesn’t have to be a flash stick. You can actually install it on your hard drive (like I do). I’ve got it under C:\FirefoxPortable\. Just attach a c: in front of the installation directory.
B) If you create/modify your FirefoxPortable.ini file (see the readme.txt file, they’ve got on all ready for you to copy where it needs to be, where your firefoxportable.exe file goes), and add
AllowMultipleInstances=true
You can then run FF 3.5 and FF 3.0(.10) together. I do it all the time. You can also do things like turn the splash screen off.
I actually took the FF 3.0.x portable version, installed the latest nightly over on top of that, and the new nightly installations update it just fine. The only problem I’ve ran into is that a couple of times, the update fails in some way, and when the portable app starts, it thinks it’s using your 3.0 profile (and hilarity ensues!). Just exit Firefox (nothing will get changed UNLESS you change it!), and restart Firefoxportable.exe. Directions to do that installation are here:
http://firefox-fangirl.livejournal.com/1977.html
I’m really about to the point where I’m ready to make my 3.5 portable the default browser. Gmail under 3.0 feels like slush anymore. I’m running Xmarks to sync everything between computers and versions even. That’s two desktops and one laptop, all running 3.0.10 and 3.5pre (It’s not even marked beta in the nightly builds now). It was a bit much trying to make sure that everything was running the same extensions. It took awhile (and I only had two computers to deal with, the second desktop is a new install), but it’s done.
Once 3.5 comes out, after making an archival copy of my portable firefox, I’ll be running 3.6alpha, just to play with.
After imaging everything to dvd, I installed Ff3.5b4 as my main browser. I had to tweak 8 of my addons by opening the install.rdf and changing the max version to 4.0. They all installed and worked fine. To date, I have had zero problems with this version of Firefox.
@True Falcon: You should have downloaded Nightly Tester as your first extension. Then you can force an install by a touch of a button. It won’t let addons.mozilla.org let you install or download those extensions, but those sites that don’t check work just fine.
I’ve been installing a new desktop, and decided to go whole hog and installed beta for real. After running into addons.mozilla.org not allowing me access, I (foolishly) installed 3.0.10, not realizing that it got installed right over my 3.5b4. I then RE-installed 3.5b4 on top of that. After getting most of my extensions loaded, my Xmarks wasn’t working, not even a little. It was DE-install 3.5b4, erase the program directory completely (there was a .dll left), erase the profile, then RE-install 3.5b4 (and I used the portable 3.0.10 to download the extensions I wanted, using Mr. Tech’s extension to install them after that). I probably could have just used another computer to download the extensions, but I wasn’t thinking that way. Everything is working fine on that system now.
The reason why I installed it? You can’t really make the portable version the default browser, because when Firefox runs without the surgery that the PortableFirefox.exe does to prepare it, it makes and uses a default profile where you don’t have one. It works great as long as you’ve already loaded it. But for the first time? Yuch.