Hi again
So I tried… Uploaded a version 1.8.1 compatible with Firefox 56+ to AMO, and as soon as it was approved I uploaded a version 2.0.2 being compatible with Firefox 78+.
When updating addons in latest Firefox version, xIFr is updated to version 2.0.2.
And when updating in an Firefox 56 based “Waterfox Classic”, xIFr is updated to version 1.8.1.
Thanks @caitlin. It seems mission has succeed 