I always like the latest and greatest so here are instructions for running the latest and greatest Firefox builds while maintaining browser auto-updated and extensions.
First off you need to download the latest night build from http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/. Alternately you can get the 3.1 Beta witch is not updated as often but still has the newest features at http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.1b1/. The second option may be more stable but it will replace your Firefox install where as the nightly installs in a separate location.
Regardless of witch version you install when done you will have a few problems, namely most ad dons will report out of date and won't load. To fix this open up Firefox and navigate to about:config. Now search for the setting extensions.checkCompatibility or add it if needed. Also set it to false. This will simply make extensions load even if their versions don't match Firefox. Now some addon's still may not work but all of my major addon's work just fine.
Also while here we want to change a few other settings. Next find the setting general.useragent.extra.firefox and set it to Firefox/3.0. This just changes the Firefox version reported to web sites so the Install Extension links in https://addons.mozilla.org/en-US/firefox still work correctly.
You should now be all set, just restart Firefox and check out some sites. Also note that extensions and settings are shared between all versions of Firefox but noting we changed will break Firefox 3.0.