Setting a proxy chain in 7.0 or Mozilla

I try to set a proxy chain in Netscape 7 -- and same in Mozilla:
The 'set proxy' command in the browse's location bar changes ':' to 
'%3A' (the hex equiv. of the ASCII number for :) like this:

http://set.proxy.192.168.1.1:1234 <enter> and the browser says "Your 
proxy-chaining configuration is:
http://set.proxy.192.168.1.1%3A1234

For info, the first proxy and port are set under
Edit>Preferences>Advanced>Proxies, in the boxes provided.

Can someone suggest a work-around?

Same problem in Mozilla 1.x as well as 2B