How to configure a firewall that only allows a few webpages and dropbox and U1 services?

Can anyone give a step-by-step how-to that explains how to

1. block all outgoing internet traffic to all websites, but allows only a selected list (needed for working, fi. online dictionaries)
2. allows Dropbox and Ubuntu One to do their job (so backups will not be lost and things get synced)
3. that can be switched off easily after a period of hard work

I have no knowledge whatsoever of networking, about hosts, IPs, subnets, subnetmasks, ports, ipconfig or whatever. In the past I always used firestarter to block all my outgoing traffic apart from a few webpages. Firestarter does not do the trick anymore. If I add an outgoing whitelist the policies just don't apply. With gufw I can't work because it is overtly complicated.

The use case: I need to stay focused on my work and thus I need easy access to the internet to be less easy. Thanks in advance on behalf of all networking noobs.

Edit/Update on why gufw is hard to use

What is hard to understand on gufw is how to define policies. In the Add Rules dialogue I do not understand

• in the simple tab, whether I should choose TCP or UDP or both. Whether I should choose a port or a service, what the dangers are of this and how to find the name of this port or service
• in the advanced tab, whether I should choose TCP or UDP or both and what the values of the from and to fields should be (especially the port number). Also why should I specify a from field? That's localhost right?

Solution 1:

This is not the first time somebody wanted to block the distraction of the internet while allowing some websites. Projects like SelfControl and FocalFilter have existed for a while. There has been a port to Linux of SelfControl and we have a few questions that travel down the same path:

• http://svn.jklmnop.net/projects/SelfControl.html
• SelfControl-like filtering
• "Get Shit Done": How to block websites temporarily?
• Timed-allowed websites with Squid/Dansguardian: Allowing a particular user to access a blocked site for a limited time