Firefox does not support explicitly binding it to a certain IP address. It uses the address selection mechanisms (for source address and destination address) provided by the operating system. Depending on your platform there are ways to change that, but you'll change it for the whole OS, not just Firefox.
Linux based systems use /etc/gai.conf
for the destination address selection. Setting a fixed source address can be done in the routing table. Take a look at the ip route add ... via ... src ...
command syntax.