Thẹn why not just leave the functionality there but force users to confirm the installation the next time you restart the browser? And make it possible to remove the sideloaded extension always?
They're converting these "sideloaded" extensions into "normally installed" ones, and then the user will gain that ability. They're not just wiping these extensions without warning.
During the release cycle for Firefox version 73, which goes into pre-release channels on December 3, 2019 and into release on February 11, 2020, Firefox will continue to read sideloaded files, but they will be copied over to the user’s individual profile and installed as regular add-ons. Sideloading will stop being supported in Firefox version 74, which will be released on March 10, 2020. The transitional stage in Firefox 73 will ensure that no installed add-ons will be lost, and end users will gain the ability to remove them if they chose to.
But then again why not just treat these sideloaded extensions as first-class citizen, add support for their removal, and add confirmation before enabling them the first time?
