It seems it still remains possible to get Chrome and Opera addons in Firefox
57 (so Abrowser57) through Chrome Store Foxified addon (MPL v2)
https://addons.mozilla.org/fr/firefox/addon/chrome-store-foxified/ as says
this french article :
http://www.papergeek.fr/firefox-comment-y-installer-des-extensions-google-chrome-opera-81008
I don't really like this option, but even if Chrome Store Foxified addon
should work on Mozilla 57, Firefox website tells me to download Firefox.
Have I missed something ?