Since I was able to successfully transfer my domains from Netfirms to DynDns.com, I’ve written to NetFirms to cancel my account and not renew any services:

Hello, I am not satisfied with the performance of your hosting service, and have transferred my domains away to another registrar. Please cancel my account "toobinator.com" and do not renew any services for it. Regards,

Disclaimer: My experience with NetFirms is not representative of all their customers. This post is not encouragement to base your hosting decision on my experience.