Each time you install a new version of Firefox and restart the browser you are presented with that version’s Release Notes. Most folks end up not even reading the notes or may be just skimming them and closing the tab. If you want to return to the notes later you could go to the Mozilla Firefox Release Notes site and then select the current version from the list. But wait, there is a much easier way to see the release notes for your current version of Firefox. Simply go the Help menu in the Firefox (or Thunderbird) menu bar and select Release Notes. Note: users who are using the nightly builds will get a page not found error.
News Source: Learn Firefox