For the last few years I have been poorly maintaining the Safe add-on for Firefox. A quick summary: it’s changes the border of a page and tab colour of secure websites - green for sites with EV certificates, blue for regular SSL, and red for broken SSL. For me, it gave a better visual cue to whether I was on a secure page rather than looking for the lock icon. It’s an addon that you don’t know you have until you feel something’s missing.
With the soon-to-be-released let’s encrypt coming, it gave me pause as to whether pervasive SSL is coming and what that means for the future of the add-on. Some people wish their projects will always stay relevant, but not me - or at least not this. A future where information is private from my machine to a server, whether it be in a trusted place like my home or in a shop’s open wifi, is a future that I would gladly put that add-on to rest.
(OTOH, maybe it would be put to rest without me. Hear about Firefox revamping their add-on ecosystem? :) )