Mozilla and Google have been working together for some time, finally integrating the Google phishing detection extension into the browser itself. By default every 30 minutes it will connect to Google's servers and download a list of new phishing sites that have been added. Then, when you go to a website, the URL is checked against that list.
Netcraft toolbar works in a different fashion, though. Instead of relying on a list of bad sites that needs to be updated it looks up the known information on the site, such as the owner, country it's hosted in, etc, to help you determine if it is legitimate.
To some degrees this offers you better protection. For instance: You are trying to sign into Ebay and get redirected to a phishing site that was just created an hour ago. The Google/Mozilla phishing detection system would not flag it since they don't know about it yet...it's not on their list. Netcraft, on the other hand, would report that the site was just created today, is owned by ILoveToHack, and is hosted in Russia. Thus, a quick glance at the Netcraft toolbar would tell you the site is fake and you should leave it ASAP.
Aside from that, I also recommend using the extension SiteAdvisor, which gives sites a green, yellow, or red rating based on spam, popups, malware, privacy invasion, illegal activities, etc. If the button turns grey it means they have no information on the site, which would not be the case for Ebay.
Personally, I enable Firefox's phishing protection but find a combination of Netcraft Toolbar and SiteAdvisor to be extremely more beneficial.
Hope this helps,
Release notes for Firefox 2.0 read "Built-in phishing protection based on the previous Google extension".
I previously had Netcraft toolbar installed. Does this new feature mean I can uninstall it?