tl;dr: We have two existing features (multipackage xpis and the
InstallTrigger api capability for installing multiple xpis in a single
call) that are not widely used. I would like to remove them to reduce
complexity in the add-ons manager.
Background: The XPI file format is used for several types of add-ons
including extensions (both webextensions and the older xul and bootstrap
ones), themes, and language packs. One of the more obscure features of the
format is "multi-package" xpis which are pretty much what you might guess
from the name: a collection of other xpis all bundled into a single file.
And then we have the InstallTrigger api, which provides a way to install
xpis from content (guarded by permissions prompts of course). This is the
api that is is invoked when you press the "Add to Firefox" button on an
addons.mozilla.org page but it is also documented for use by others. This
api is an ancient one that we inherited from Netscape (!!) though it has
never been standardized or implemented in other browsers. Among other
things, InstallTrigger provides a way to install multiple xpis with a
single api call.
Unfortunately we don't have any telemetry available about how frequently
these two features are used but empirically they appear to be very
uncommon. However, they are responsible for a good deal of complexity in
the add-ons manager and, more urgently, they complicate the UX design for
current efforts to display detailed permissions prompts to users when they
install a webextension.
Rather than deal with the UX design and corresponding implementation
complexity of install operations that include multiple webextensions that
need to be individually approved by the user, I'm proposing that we drop
support for multi-package xpis and restrict the InstallTrigger api to only
allow installing a single xpi at a time. I would like to do this in
Firefox 53.
-Andrew
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