If you like living on the cutting edge of software, you’ll want to download Firefox 4.0 beta 1.
Those keeping track of such things will know that Firefox 4.0 was previously known as 3.7. That aside, there are some cool new features available in this release:
For any doubt, release notes are here: http://www.mozilla.com/en-US/firefox/4.0b1/releasenotes/
About add-ons, it says:
Add-ons installed with previous versions of Firefox may not yet have been updated by their authors to work with Firefox 4 Beta. If you wish to help test Add-ons, please install the Add-on Compatibility Reporter - your favorite Add-on author will appreciate it!