Mozilla has promised that the will provide a way of automatically signing extensions, so that programs like my own will be able to sign the extension automatically. But also warned that this will not be available when the first versions of Firefox that will require are released. So for some time all extensions made from it will not install.
While waiting for an automated system to sign the generated extensions there are some work arounds. One is not to upgrade past Firefox 39 for awhile. I don't recommend this as you will miss out on important security updates. Other options are to move to another supported browser like Pale Moon or SeaMoneky which though similar to Firefox, will not require extension signing (yet). Mozilla has also promised an unbranded version of Firefox with extension signing disabled, but details on that are still lacking.
To help I have started to upload some of the popular buttons to addons.mozilla.org. You can find them on my profile page.
Some people might be tempted to upload the buttons as unlisted extensions on AMO. I ask that you kindly don't unless you first change the extensions ID, or else you will cause problems for me later.