Mozilla announced today that add-ons that depend on XUL, XPCOM, or XBL will be deprecated and subsequently incompatible with future versions of Firefox:

Consequently, we have decided to deprecate add-ons that depend on XUL, XPCOM, and XBL. We don’t have a specific timeline for deprecation, but most likely it will take place within 12 to 18 months from now. We are announcing the change now so that developers can prepare and offer feedback.

In response to this announcement, I’ve taken the step of discontinuing all of my Firefox add-ons. They all depend on XUL or XPCOM, so there’s no sense in developing them for the next year only to see them become non-functional. AutoAuth, Comment Snob, Feed Sidebar, Links Like This, OPML Support, RSS Ticker, and Tab History Redux should be considered unsupported as of now. (If for any reason, you’d like to take over development of any of them, e-mail me.)

While I don’t like Mozilla’s decision (and I don’t think it’s the best thing for the future of Firefox), I understand it; there’s a lot of innovation that could happen in Web browser technology that is stifled because of a decade-old add-on model. I only hope that the strides a lighter-weight Firefox can make will outweigh the loss of the thousands of add-ons that made it as popular as it is today.

http://www.chrisfinke.com/2015/08/21/my-future-of-developing-firefox-add-ons/