Downgrading from Firefox 57 to 56 could break things and data loss can also happen, but Mozilla has taken measures to avoid these, so going back to earlier version from Firefox Quantum, may not cause any issues to you.

After Firefox 57 release on November 14, Firefox users on 56 will be upgraded to version 57. If they don’t like the rectangular tabs ,which Photon UI brings or the new UI or the fact that their old legacy add-on no longer work can eventually make them revert to Firefox 56.

But doing so may break storage and these subsystems: Quota Manager, IndexDB and Cache API. Mozilla is aware of this, it has done tests and patched Firefox 57 (currently in beta) for this not to happen.

How to downgrade from Firefox 57 to Firefox 56

  1. Uninstall Firefox 57.
  2. Control Panel > Programs > Programs and Features > Select Firefox 57 and click ‘Uninstall’.
  3. After that, visit this page and download and install Firefox 56 version.
  4. That’s it! You’ve downgraded to Firefox 56 from 57.

Note: Do remember, using userChrome.css file offered by Classic Theme Restorer you can still able to modify Firefox 57 UI.