Drag-drop .XPI Files onto Add-ins Sidebar Fails to Install #155
Description
Drag-dropping an .xpi addon file onto the "Add-ons" AiOS sidebar (eg. from a folder other than the profile's extensions folder) for an add-on not currently installed just results in the addon showing up (hidden at the bottom, shown if filter for it) with "Downloaded" status forever, as shown in below screenshot, failing to ever install or even copy it to the profile's extension folder. After restart, even that addon item with Downloaded status in the Addons Sidebar disappears.
Because Addon Manager buttons mostly redirect to the Add-ons Sidebar instead of ever showing the full Add-ons Manager, a user would need to be aware of that the issue applies only to AiOS's version and then know about the about:addons page, visit it, and then drag-drop the XPI file onto that instead in order for addon installs to work.
Reproduced with Firefox Developer Edition (x64) v55.0b2 and All-in-One Sidebar v0.7.31.
