Mozilla is finally making it so extensions are not enabled by default when in Firefox Private browsing mode. The move increases security by preventing extensions from tracking user behavior while in a mode that users expect the greatest degree of privacy. This is useful for those who wish to use an ad blocker or userscript extension, while benefiting from the enhanced privacy of the private browsing mode in Firefox. The new setting does not mean that it is not possible to run extensions in Private windows, but that you have to enable the specific extensions that you wish to run.
Source: https://www.bleepingcomputer.com/news/software/mozilla-finally-nixing-extensions-in-firefox-private-windows/