Thank you Afnan Khan, and good people at Mozilla.
I have not tested this extension yet because I still mainly use a version older than FF57("the Great Regression"), but give it 5 for encouragement. Every user of FF that uses more than 10 tabs in a session, needs an "auto unload tabs" functionality, like we need air.
After I have searched(2018) and read about/viewed screen shots (videos), of extensions that are able to unload(auto) tabs, I find this one to be the closest with the extension:"Auto Unload Tab by def00111" which does not work on FF after v56, but does a superb job on FF before 57, of unloading(auto too) tabs from multiple groups without using much CPU.
Hopefully Afnan Khan did not quit maintaining(or improving) this much needed add-on.
About the loosing of favicons of tabs it seems to be a bug of Firefox ("Most notably, tabs may loose their icons or display as loading. Fixing that is Mozillas job")...see more here: https://bugzilla.mozilla.org/show_bug.cgi?id=1450382
https://bugzilla.mozilla.org/show_bug.cgi?id=1450371