If you are using XP SP2 there are some tweaks
here that might be useful- make sure you are comfortable with editing the registry though. SP2 limits the number of simultaneous TCP connections to 10 and this can cause problems if you use p2p software.
There is nothing that you can do to the modem to change performance- any firmware changes are done from the ntl side, but the modems can easily handle 2/3 meg.
If you are looking to tweak the connection then you need to look at your pc more than the modem. As others have mentioned:
- check for spyware/viruses
- disable p2p software when you are not using
- check MTU size
- use any browser tweaks (such as the firefox ones that you have)
- eliminate any firewall software
- if you are using a router, make sure that the MTU on the pc and router are matching (or the router is set to automatic)