Works, but... Rated 3 out of 5 stars
Well, it technically works. Protected tabs cannot be closed.
HOWEVER, they also give no indication that they are protected--there's no protected tab symbol and the close tab button remains on the tab.
This is just a nuisance, but it is a pretty major one, as nuisances go.
Also, support for middle-click protecting (overriding the default function of closing the tab) would be helpful. Accidental double clicks are easier than accidental middle clicks, resulting in unwanted protection of an open tab--which isn't noticeable until you try to close the tab since there's no indication that the tab is protected!
Overall, I guess this is better than accidentally closing a tab I want open, but not by that much--now, instead of accidentally closing a tab I want to keep open, I run the risk of accidentally protecting a tab I want to close.
Once these issues are fixed, I'll update my rating.