Can't get this to work at all...
Browser Extension can either be installed or uninstalled in Avast (not disabled), created the .xpi as mentioned. Removed the 'dead' AOS in FF51.0.1 and restarted FF.
Dragged the renamed .xpi into an empty tab and it installed, but every thing reported in my Reply #15 is still present. Add-on enabled, but AOS icon greyed out, no results in searches or on clicking the AOS icon.
Tried all sorts of options/ideas, uninstalling the Security browser extension before drag and drop of .xpi, installing (no disable/enable) of the Security browser extension. No matter what I tried even though AOS was installed, it simply didn't work.
I think because of the lack of a disable/enable in the AvastUI Security browser extension every time it is enabled (install Component) it undoes everything that has gone before.
Not sure this chase is worth the effort, it will remain uninstalled now.