Mozilla has released Firefox 55 onto its FTP servers but users won't be offered the update from the Firefox website or via the in-built updater until August 8. The release is pretty big and includes WebVR, a new setting to fine-tune your browser's performance, and much better responsiveness with Quantum Flow.
Some other new features in this release include the ability to move the sidebar to the right hand side of the browser, the ability to take screenshots of webpages (currently undergoing A/B testing so you might not see it), the Belarusian locale has been added, it's now possible to assign custom shortcuts to Firefox menu items on OS X and macOS via System Preferences > Keyboard > Shortcuts, the print preview has been simplified, search suggestions are now on by default for users who haven't explicitly opted-out, and, finally, you can search with any install search engine directly from the location bar.
With Quantum Flow, users with a lot of browser tabs could see start up improvements up to 30 times faster than they saw in Firefox 51. Quantum Flow also reduces the amount of RAM being used by the browser, with a Mozilla developer saying Firefox 30 with 1691 open tabs used 2GB of RAM where as Firefox 55 cut this to under 500MB of RAM (page contents weren't loaded in either instance).
If you're on the Extended Support Release, you should receive an update to version 52.3 with security fixes backported from Firefox 55. If you do upgrade to Firefox 55 you should be aware that your Firefox profile will be incompatible with older versions of the browser. There's a small chance that Mozilla will discover an issue with Firefox 55 between now and it's official release tomorrow so be careful if you go digging through the FTP server looking for the upcoming release.