Circuit isolation by SOCKS proxy may be breaking other proxies or non-proxies
If, for whatever reason, the user wants to disable the Tor Browser from connecting to their local Tor daemon (such as to prevent tor-through-tor with forced transparent torrification, or use the Tor Browser for it's security rather than it's anonymity, or point the Tor Browser to an SSH SOCKS proxy), they will find that since the release of the Tor Browser 4.5.1, it will fail to connect to any SOCKS proxy that isn't a Tor daemon, and it will complain saying, "Firefox is configured to use a proxy server that can't be found." when proxies are disabled.
I believe the cause of this issue is https://gitweb.torproject.org/torbutton.git/commit/?id=59445b7baed58e712bd38c02e6dc75882ff0c997 or a related commit to the Tor Button. Disabling the Tor Button resolves the issue, but this isn't desired as the Tor Button provides additional protections such as the HTML5 canvas extraction.
This can easily be recreated by having the 32-bit Linux package of the Tor Browser, and the attached 0000tor-use-system-4.5.2test.js file in the current directory, and then running something along the lines of:
xz -d < tor-browser-linux32-4.5.2_en-US.tar.xz | tar -xv cp 0000tor-use-proxy-4.5.2test.js tor-browser_en-US/Browser/TorBrowser/Data/Browser/profile.default/preferences/ tor-browser_en-US/Browser/start-tor-browser --verbose
The above assumes there is a SOCKS5 proxy running on your machine on port 1080. This can easily be done with something like
ssh -Dlocalhost:1080 localhost -N