Open Ports

[John]

Hi.

My laptop is running Ubuntu 8.1 with Gnome (all current automatic 
updates installed).

I am using Thunderbird for my email and I have downloaded the two 
WebMail extensions to allow me to read various hotmail accounts.

However, every time I start Thunderbird, it reports "Could not connect 
to server localhost; the connection was refused". (When I used to use 
the identical configuration on a Windoze XP machine, it worked.)

My laptop is connected to my SuSE 10 server (as was the XP machine) and 
I know that its firewall to internal computers is fully open and also 
that nothing has changed in its configuration whilst I transferred from 
XP to Ubuntu.

Also, Thunderbird talks quite happily with the Postfix system on the server.

My main question at the moment is how can I determine which ports are 
open on my laptop (I have not configured a firewall on this machine!)?

In effect, the WebMail add-in to Thunderbird is complaining about ports 
110, 25 and 143, some (or all) of which must be open to allow T/Bird to 
work in the first place!

Hoping for some good pointers, please!

TIA

J