The new extension engine shipped to the general public as part of Firefox 57 in November 2017.
While magicalbeans may be a bit hyperbolic in their criticism, as a web developer who uses all the major browsers, I am still annoyed every day by little things that are worse in Firefox since that time. It is a reasonable and valid criticism that many useful extensions were broken by the change and that a significant part of the functionality that was lost can't be reimplemented in the new system. The claims that the general reduction in functionality through loss of previous extensions would be a temporary problem and would be corrected as the new model evolved over subsequent versions have proven to be optimistic.
It is also fair to say that claims of extensions being better contained and more stable in the new model have been exaggerated. I see far more problems caused by the smaller number of extensions I now use than I ever saw before, and I have done consistently ever since the change, through three more major versions of Firefox itself and several updates to most of the extensions.
I appreciate the intent to make Firefox faster and more reliable and more secure. Surely no-one would argue that those aren't good things. But the fact is, a lot of stuff did get broken and hasn't been fixed, and for the class of users who valued Firefox for its customisability, it is a worse browser today than it was 9 months ago.