Lemon Foundation

Many developers, including myself, use Firefox as our browser of choice.  This is partly due to the massive number of extensions available, including some which are extremely useful to developers (like the developer toolbar, Firebug, etc) and generally useful others, such as AdBlock+ and Tabs Mix Plus.

The problem with this is that it can cause the browser to become quite slow and clunky, not to mention that Firefox already has issues with memory usage.  Extensions like AdBlock+ also modify the page, which can result in cases where developers are no longer seeing what clients and users are seeing, making it harder to identify and debug problems.
It's a good way to keep a lightweight 'vanilla' version of Firefox to hand for testing purposes, and it even supports extensions, so things like the web developer toolbar can safely be installed to debug problems. though I'd recommend keeping away from installing too much as it's best to keep it as light & quick as possible.

I've found it very helpful to keep my regular browser and 'development' browser seperate, not least because I'm an extension junkie, and install/uninstall many extensions in my regular Firefox each week!

There's many other common applications ported to portable versions (so that they can be used without requiring installation, making them ideal for carrying around on a portable USB drive).  Check out PortableApps.com for the full list.