Bobbert,
Firefox 57 introduces WebExtensions which disable most [older] Firefox extensions.
If you have been modding your Firefox profile for the last 10-15 years, chances are good to excellent this destroyed your profile. Your extensions will never work again and your user interface mods will not work on Firefox again.
This was done for security and compatibility reasons among other reasons. People who pushed to have users, *choose* compatibility coexisting at the expense of security, lost. We will see what inpact this has on user base numbers. It seems not much.
*Before* leaving Firefox, consider that your extensions will continue to work on Firefox ESR (same as Firefox 52 but updated with security patches unlike regular Fx52) until June of 2018.
https://www.mozilla.org/en-US/firefox/o ... tions/all/
That would give you more time to see what to do in second half of 2018.
Firefox ESR 52 is fully compatible with Firefox 54 profiles or earlier. So if you have a backed up profile from Fx54 or - great, use it!
But if your profile has been touched by Fx55 or higher, you need to create a new one.
There is an extensive way to do this correctly but if done "quickly":
1. Copy /paste the entire CHROME folder from Fx55/56/57 to Fx ESR
2. Cooy/paste user.js file?
3. Copy/pasting the places.sqlite bookmark file
4.
https://addons.mozilla.org/en-US/firefox/addon/febe/
saves your already installed extensions into .xpi installable files with actual file names that are named after the extension names and not random numbers and letters like when you save profile extensions the regular Firefox way. That way you can quickly transfer extensions.