Profile Switcher Extension for Firefox

The ProfileSwitcher extension enables you to use several profiles in Firefox and Thunderbird.
So you could create a developer profile and install all programmer extensions (like Firebug, Web Developer toolbar, etc.).
But if you just want to navigate, you don't need them and loading all of them just slows down the startup time of Firefox. Just use another profile without them, with a different set of extensions.

From the extension's preferences, you can choose what to do when you launch another profile (close the one in use, don't close it, ask every time).

Install instructions

Unfortunately, this extension can't be found on Mozilla yet, so just download it from the home-page or use this link to download the recent one when this post was written.