Firefox Blocks Autoplaying Web Audio (engadget.com)
Mozilla's latest Nightly builds for Firefox now include an option to mute autoplaying audio. The feature was recently added to the Chrome browser, but Mozilla's update offers a few more options. According to Engadget, "You can turn the feature off entirely, force it to ask for permission, and make exceptions for specific sites." Keep in mind that these are nightly releases, so you will most likely run into some bugs. The "polished version" is likely weeks away.
I use a lot of strange addons, and nearly every one was available immediately or just a few months after the switchover.
That's a big "nearly". There's no counterpart to Keybinder for Firefox 57 and later, and there won't be until bug 1325692 is fixed.