Firefox 52:
Release channel - drops support for NPAPI plugins except for Flash (Java, Silverlight, etc)
(ESR channel will retain NPAPI plugin support)
https://www.bleepingcomputer.com/new...-except-flash/

Firefox 53: (scheduled for release on April 18, 2017)
Drops support for XP and Vista

Firefox 57: (scheduled for release on November 28, 2017)
Drops support for XUL and XPCOM extensions

Firefox is transitioning to "web extensions". These will be similar to Google Chrome extensions. Note that web extension support has been gradually added since FF48.
http://www.ghacks.net/2016/11/24/fir...e-end-of-2017/
https://wiki.mozilla.org/WebExtensions/FAQ

Extensions that affect the UI, like Classic Theme Restorer, will no longer work.
Old extensions that are not maintained and/or ported to web extensions won't work either.
If an extension has a version that works in Google Chrome, it will probably be ported over to Firefox web extensions.

TLDNR: If you need to use XP or Vista, NPAPI plugins, or XUL/XPCOM extensions, you should transition to the ESR channel.
The easiest way is to just install the ESR version over the top of the Release version.

To manually change to the ESR channel, edit the channel-prefs.js file:
Make sure Firefox is CLOSED first
Open the Firefox program folder (Ex. C:\Program Files (x86)\Mozilla Firefox)
Open the defaults\prefs folder
Open channel-prefs.js with Notepad
Change pref("app.update.channel", "release"); to pref("app.update.channel", "esr");
Save channel-prefs.js