Artem Tashkinov writes: Mozilla has published a plan of add-ons deprecation in future Firefox releases. Firefox 53 will run in multi process mode by default for all users with some exceptions. Most add ons will continue to function, however certain add ons have already ceased to function because they don't expect multi user mode under the hood. Firefox 54-56 will introduce even more changes which will ultimately break even more addons. Firefox 57, which will be preliminarily released on the 28th of Novermber, 2017, will only run WebExtensions: which means no XUL (overlay) add ons, no bootstrapped extensions, no SDK extensions and no Embedded WebExtensions. In other words by this date the chromification of Firefox will have been completed. If you depend on XUL add ons your only choice past this date will be Pale Moon.
... when fits of creativity run strong, more than one programmer or writer
has been known to abandon the desktop for the more spacious floor.
-- Fred Brooks