One way to access an External Proxy behind an Internal proxy is to SSH External Proxy (if remote SSH enabled) and bind the connection to a specific port in your system. Thereafter, use the port and 127.0.0.1 as Network Options (Socks5) for Firefox.
If the system you've is Linux, type in Terminal
ssh -D Port_no user@server_address server_port, where,
- Port_no is the binding port you can select for the system, try a port greater than 1024;
- user is the username provided by external proxy;
- server_address is the IP address of the external proxy;
- server_port is the port where the Server is receiving the SSH connection;
For Windows,many prefer Putty, but I would not be able to assure how it will work.
About Putty :
PuTTY is an SSH and telnet client, developed originally by Simon Tatham for the Windows platform.
Link for Putty : Here.