C'est exactement ce que je cherchais.

When a FF57 version will released, my old sweet proxy switcher will come dead. ;(
I find this plugin suitable for me, if not for a few unfortunate things in it.

I tuned my FF with my own privacy, so this plugin did not save proxy settings. Setup disappearing after browser resart.
Okay, I found two values (about:config) that affect this.
First, "network.cookie.lifetimePolicy=2", as known as "Delete cookies after shutdown" do not allow to store settings, so proxy settings lives just until I close my FF.
Second, "privacy.sanitize.sanitizeOnShutdown=true", as known as "Clear browsing history on shutdown". This value kill all proxy settings after browser shutdown.
Can developers organize new independent storage, instead webappsstore.sqlite affected file?
And some feature requests:
1. Need setting "Do not use any proxy for...", "use system Proxy" options.
2. How about ftp-, ssl-, socks-, "do not use proxy for..." settings, same as in standard connection dialog menu?