Mozilla will release Firefox 21.0 on May 14, 2013 and shortly thereafter update the Beta, Aurora and Nightly channels of the browser to Firefox 22.0, 23.0 and 24.0 respectively.
The updates will be transferred to Mozilla's ftp server first before they will be announced on the official website. If you have configured automatic updates, you should not have to worry about that though as your browser will get updated automatically when you start it after the update becomes available.
Firefox 21 What’s New
We rely on the beta release notes - as always - as our main source to find out what is new in this version of the Firefox web browser.
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