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.

>>10
The effort depends on what kind of browser
1.No graphics, plain text,no js/css. links-tier, a few weeks.
2.Graphics/Video/Sound/etc, no js/css a few months.(more if rare formats and latest standards)
3.same as #2 with css probably 4-6 months CSS3 parity
4.JS interpreter. 1 year(for decent speed and WebGL 1.5-2years)
Keep in mind it will be filled with bugs/incompatibilities, used by almost nobody and will be slower than everything else.
Browsers today are essentially "Virtual machines" for content, not "Web parsers/viewers".