After turning off Sync Clippings, the old Synced Clippings folder incorrectly retains its special icon

[aecreations]

This is a leftover issue caused by the fix for issue #243. The Synced Clippings folder incorrectly retains its special folder icon in the Clippings menu after turning off Sync Clippings, but the user has chosen not to remove the folder.

There is a solution: from the Mozilla Add-ons Blog: https://blog.mozilla.org/addons/2018/11/08/extensions-in-firefox-64/

browser.menus.update() now allows extensions to update an icon without having to delete and recreate the menu item.

However, this requires Firefox 64 or newer, hence the deferral of a fix for this issue to post-6.1, as version 6.1 needs to be compatible with Firefox 60 ESR.

[aecreations]

It is not desirable to force a menu rebuild after turning off Sync Clippings, since a rebuild will happen again after removing the Synced Clippings folder (if the user has chosen to leave the default option to do so). Two menu rebuilds in quick succession can cause menu corruption problems.