Major Changes are coming for Firefox 52, 53, and 57

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

Does the ESR look and act just like regular FF?

ESR stays on a certain version and only releases security updates. The current version is based on FF45. The next ESR will be based on FF52.

https://www.mozilla.org/en-US/firefo...nizations/faq/

I'm using Win 7 now anyway.

And all extensions will be lost after v.56. So I'll probably move to SeaMonkey or ESR 52 anyway:
http://forums.mozillazine.org/viewto...?f=7&t=3027638

Not ALL extensions; XUL and XPCOM based extensions. Basically, they are switching to web extensions which are going to be more like Chrome.

Sorry about the error. :(

I think Chrome has a total of 30 extensions,some of them don't work, and their UI is naked. I used to like Opera as a fallback, then they redid it to look like Chrome. No, thanks.

Will I lose all the code tweaks I've done in the chrome folder, or will I just be unable to make new ones?

Google Chrome has hundreds of extensions, so I'm not sure where you got 30 from. Opera is now based on the same Webkit engine that Chrome used before (Chrome now uses Blink engine which is a fork of Webkit).

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Will I lose all the code tweaks I've done in the chrome folder, or will I just be unable to make new ones?

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Things like the userchrome.css won't work because Firefox won't have the XUL or XPCOM code in it. They mention it in the link you posted.

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In the case of tweaks using user.Chrome.css, they will be gone.

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From Train:

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The bad news for Firefox users continues with the fact that the upcoming Firefox 53.0 release won't work on Linux computers powered by processors older than Pentium 4 or AMD Opteron, and 32-bit Mac OS X operating system won't be supported as Firefox is now distributed as a 64-bit only app for macOS Sierra or later.

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http://news.softpedia.com/news/firef...n-513716.shtml

32-Bit Firefox Users on 64-Bit PCs Will Be Auto-Upgraded to 64-Bit Firefox
Mozilla engineers have decided that starting with Firefox 56 they will silently and forcibly auto-upgrade some users of 32-bit Firefox to 64-bit versions.
https://www.bleepingcomputer.com/new...4-bit-firefox/

If you prefer the 32-bit version, this is another reason to use ESR.

If you haven't migrated to ESR yet, now is the time to do it.

https://www.ghacks.net/2017/08/07/fi...t-what-is-new/

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You cannot restore an older version / migrate to Firefox ESR after the upgrade to Firefox 55.

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I have FireFox 54.01 and it's still 32 bit. Why didn't it change to 64 bit with the upgrade?

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starting with Firefox 56 they will silently and forcibly auto-upgrade some users of 32-bit Firefox to 64-bit versions.

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From post 10.

Thanks Train. I missed that altogether.