the guess work
If I understand you correctly you want to connect from your client with a browser to server2, which then tunnels the connection to server1 and thus allows you to access the office net via the squid-proxy installed on server1.
just to get the guess work complete I think you used something like
server1$ ssh -R 12312:localhost:22 server2
to setup the first tunnel.
a solution attempt
if you really want to be able to enter server2 as a proxy in your client's firefox settings make sure that in server2's sshd_config (/etc/ssh/sshd_config) GatewayPorts is enabled:
also make sure that no blocking iptables rules are in place for the ports you want to use.
then instead of your existing tunnel you could use:
server1$ ssh -R *:3128:localhost:3128 server2
or if you really want to use your existing tunnel you could use:
server2$ ssh -L *:3218:localhost:3218 -p 12312 localhost
some side notes
If you do it in the way explained above everyone who can reach server2 (no login required) can use port 3128 as proxy port and thus access your office net (if no other measures are in place)
I don't know what kind of policies exist in your company but before attempting such tunnels make sure you don't break any.
change your existing tunnel to
server1$ ssh -R 3128:localhost:3128 server2
from your client issue a
client$ ssh -L 3218:localhost:3128 server2
then in your firefox add localhost and port 3128 as proxy config.
like this you don't need the configuration of GatewayPorts and only people with a login on server2 can use the tunnel to server1