Rated 5 out of 5 stars

Version 43.0a2 (2015-09-27) of Aurora blocks it.

IS THERE ANYONE AVAILABLE TO MAINTAIN THIS ADD-ON?
IT MUST CONFORM TO SOME PRESCRIPTIONS OF THE MOZILLA PEOPLE.
(see my other post below)

It works (... with a user-developed workaround) !!
And it serves exactly what is needed by anyone who uses several Gmail accounts.

Within 1 or 2 months (which is the time to get first the beta version and then the official Firefox release) the blockage will reach the other releases of Firefox.

EXPLAINATION:

Starting with FF41 it will be compulsory that the application be "signed", meaning that the developer must conform to some specification: see
https://developer.mozilla.org/en-US/Add-ons/Add-on_guidelines

Otherwise the application will be DEACTIVATED:
https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox

It will not be possible to bypass this deactivation, because it will be hardcoded in FF.

Since
all actual users of this add-on are relying on a fix informally written by another user,
and
the original developer (Todd Long) is no more developing the add-on,
and
the developer of this fork (Erik Nedwidek) is also no more developing the add-on:

It is most probable that starting from FF41 noone will be able to use this precious add-on.

Is there anyone being able to maintain/solve the problem/develop again the add-on?

This user has a previous review of this add-on.