Firefox addons will now be only on WebExtensions

https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/

By the end of 2017, and with the release of Firefox 57, we’ll move to WebExtensions exclusively, and will stop loading any other extension types on desktop. To help ensure any new extensions work beyond the end of 2017, AMO will stop accepting any new extensions for signing that are not WebExtensions in Firefox 53. Throughout the year we’ll expand the set of APIs available, add capabilities to Firefox that don’t yet exist in other browsers, and put more WebExtensions in front of users.

I don't even know what the difference between XUL and WebExtensions is. How bad is this? Does everything have to be written again from scratch?

I only use Firefox because of the addons. I don't want to lose Pendatactyl or Rikaichan.