The previous minimum version of 56.2 resulted in the build failing
validation when it was uploaded to the Firefox web store, because that
version doesn't exist. It was set to that version because a Firefox
fork uses it.
Instead the minimum version has been reduced so that we pass
validation. Unfortunately this will mean that a single incompatible
version of Firefox Mobile will allow the extension to be installed
(in theory), but there was no other way to avoid cutting off support
to WaterFox (the Firefox fork). The warning about this from the addons
linter can be ignored for now.
The minimum compatible version of Firefox has been lowered from `60.0`
to `56.2`. It was originally set to `60.0` because that is Firefox ESR,
which currently the minimum Firefox version with security updates.
However Waterfox is based upon Firefox `56.0`, and has backported
security fixes from the ESR release.
This change in minimum version requires no additional transpiling, and
doesn't affect any browser APIs we use.
It does introduce one additional warning in the `addon-linter` about
Firefox for Android `56` lacking support for browser action popups.
However there is no version `56.2` of Firefox for Android, so the
minimum version remains `57` in practice (which does support browser
action popups).
The minimum version supported is now Firefox 60. This is the current
Extended Support Release. Various features we use were not supported by
Firefox 53, such as `browser_action.default_popup`, `tabs.query`, and
`permissions:unlimitedStorage`.
Set the minimum browser version supported in the extension manifest.
Currently we only ship the extension on Chrome and Firefox, so the
minimum version has been set for those two browsers.
Relates to #6805
