Who creates these add-ons?

The add-ons listed here have been created by thousands of developers from our community, ranging from individual hobbyists to large corporations. All publicly listed add-ons are reviewed by a team of editors before being released.

The issue with incompatibility with latest version of Firefox, as the developer to update the extension(add-on)

The best way is contact the developer directly

• https://addons.mozilla.org/en-US/firefox/addon/download-scheduler/

I understand. But Firefox updates faster than the developers can keep up. I was hoping for a solution for v28.

Preferably one that will last a few version upgrades.

Firefox updated every 6 weeks once.

Check this

• https://hacks.mozilla.org/2012/05/firefox-and-the-release-channels/

Yes there is an alternative method for scheduling download in Firefox. I have found this useful topic - How to Schedule Downloads in Mozilla Firefox?

Hope this may help you.