I made the mistake of letting Firefox update itself to the latest version, and so I lost all my valuable extensions. After trying it for a while, it was obvious to me that I had lost a lot of functionality, so I decided to revert back to version 56.0.2 (latest stable version).
So, if somebody like me, wants to revert back to v.56 without uninstalling and reinstalling a fresh copy, and then prevent automatic updates to v.57, please continue reading.
After looking around to find the best solution to revert back, it seems that the old version can be safely installed on top of the new without losing any settings or add-ons (based on this post).
So, the steps followed were (am using Windows 10 64bit):
Made a back-up of my profile folder based on this just in case
Downloaded the 64 bit version from filehippo: Firefox v56.0.2 64bit (the 32 bit version is here filehippo: Firefox v56.0.2 32bit) and reinstalled v.56 direct on top of version 57 without previously uninstalling it.
Before restarting, I disabled my internet connection (just to prevent Firefox from self-upgrading) and then after launching Firefox 56, I changed the update option as follows
Tools --> Options --> General --> scroll down to "Firefox Updates" and check "Never check for updates (not recommended)" - Other options proposed in this topic might work as well.
Shut down and Restart computer (this time with internet connection enabled as usual) and launched Firefox 56, which had all the settings, extensions and add-ons intact as before upgrading.
I am using v.56.0.2 without problems for a week now (of course I don't touch "check for updates" in Help --> About Firefox). In case that for any reason, Firefox upgrades itself again to v57, you can repeat the above procedure. Happened to me with my laptop.
Although I know that I will be left without updates for a while, my decision is to wait until all my critical add-ons and extensions are upgraded for v.57 (and all the "child diseases" of the new version are eliminated).