One of new feature from this release is Add-ons Manager. Add-ons Manager doesn’t open in a new dialog box but it opens in a new tab in the same browser window. It shows a better organized interface compared to the old one. The fonts size has been increased for better readability.
You can also check release notes here
Remember it’s alpha version not final version. You should install the software for testing only. If you want to use Firefox as your default browser please use Firefox final version.