You can find the Firefox release archive at https://archive.mozilla.org/pub/firefox/releases/. The release before Quantum (57) was 56.0.2, choose the correct folder. Then choose the correct folder for your platform. Then choose the language you want. For example, if you want Windows 64 Bit in English, choose https://archive.mozilla.org/pub/firefox/releases/56.0.2/win64/en-US/Firefox%20Setup%2056.0.2.exe. You may need to uninstall the current Firefox.

Software is in a constant flux and will always be subject to change. Downgrading is not a long-term solution, because you will not only miss new features but also will be more vulnerable to attackers because of security holes yet unknown.

First, Type about:preferences#advanced<enter> in the address bar. Under Advanced, Select Update.

[v57+] Type about:preferences#general<enter> in the address bar. Select Update.

Select Never Check For Updates.

Also turn off Use a background service to install updates

And how do we restore all our carefully customised addons if we have to reinstall???

Wouldn't it just be easier to be able to stop a mozrestored version from autoupdating every time it is restored?

Firefox comes in two or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user folders would not be affected.

Thank you :-)

I got brave. Just closed firefox, and installed firefox setup 56.0.2 (the last version before quantum). EME free over the top of v52esr. I also installed it over the top of quantum (v57) the same way. Don't need to turn off wifi. Make sure updates and background service both turned off in tools\options before installing, or it will immediately update again! Download at https://ftp.mozilla.org/pub/firefox/releases/56.0.2/

Dont instal v52 ESR. Onetab didn't work.

From now on I will mozbackup, with updates set to prompt, before I instal any updates!

Colourful tabs and decentraleyes both needed to be downgraded as v57 had updated them. Colorful tabs site advised that legacy version can be downloaded from https://addons.mozilla.org/en-US/firefox/1368/history/ but that is a dead link; correct link https://addons.mozilla.org/en-US/firefox/addon/colorfultabs/versions/ . Search for "56" to find the last compatible version for v56.

Webpaint no longer working. opens browser console instead of dialog. Successfully reinstalled from my download folder (where I saved most addo-ns when downloading)

HELLO Onetab! my tabs all returned. My tree tabs all working. My statusbar back, and context menus all in place. After 2 rollbacks all 33 legacy add-ons working perfectly, my customised settings were still there. Happy me. Thanks viciouspecan!

(I installed the 64 bit eme free. The 32 bit eme free doesnt seem to support java- have to use fb mobile version https:\\m.facebook)

Update 7 Dec 2017: After backing up the new v56, my taskbar icon reopened v52! Before closing your new v56, if there is a new taskbar icon firefox(2) make sure that is pinned.

==

I will be waiting till quantum can carry over these settings (screenshot):

As to the add-ons that updated, go to their Mozilla add-ons web site. Near the bottom is a link to older versions of that program.