Perfect but WARNING 20.0.1 !! Rated 5 out of 5 stars

Perfect.
[SOLVED] WARNING : doesn't work on V20.0.1 !! (icon & caption do not appear in modules bar) --> Customize your add-on bar ("barre des modules" in French) to add Proxy Selector icon as it disappears !

This review is for a previous version of the add-on (0.92.00). 

I just tested it in a fresh profile and I doesn't have any issue!

Could you please let me know more information about it?
Do you have any report in console?

My contact is michele dot pezza at gmail dot com.

Thanks in advance for your help.
FD

P.s.:
Maybe I understood...
you are French so with "modules bar" you mean "addon bar"...

I removed the code to manage the old statusbar
and now there isn't a check box in the options panel to show it.

To show the button in the addon bar, you can move it there
using the standard customize of Firefox.