Let's assume you're a very "privacy conscious" person who only accepts cookies for sites where you feel they are necessary -- say ones where you're going to login, or you really want to read articles there and you can't without the cookies, or whatever. Under Firefox (and Chrome under the old modality) your cookie setting choice was "Block but notify on new cookies."
Under the old model, when you first tried to access that site, create an account, login, register, etc., you'd get the initial pop-ups that you needed to respond to, that made it very clear that there were cookies involved now that you might want to accept. This is in fact a modal decision, because not accepting those cookies at that point will have consequences (like registration sequences that keep repeating, login prompts that won't accept your input, and so on).
Now the new model. As you browse the Web the little cookie icon is constantly popping up in the bar. Sometimes it shows clear and sometimes it shows blocked -- but after a while you're just going to ignore it as you go flying from page to page. There's nothing in that icon to alert the user that they've reached an important decision point about an initial cookie from a site. Even if they think to click that icon at the right moment on a new site, they have
to do more clicking to dig down into the cookie management system to accept it if they wish to.
Old model: You're on a page where you want to login. You get a pop-up that there's a cookie. One click on Yes. Finished. Easy to do, and impossible to miss that there's a key decision point.
You really do want people to make a go/no-go decision on initial cookies from sites, and not create a situation where they can easily go winging by those initial cookies and have them fall into a default blocked state -- since the consequences of doing this are a mess and require going in and deleting cookie blocks manually.
It's really initial presentation of first cookies on a new site (when the user is defaulting to blocking cookies) that is the major concern. In that situation, the user should be presented with a modal choice so that they cannot easily miss the fact that they are at an important "exception" decision point -- that is, accepting a cookie when their
default is not to accept all cookies.
And remember, by not choosing the simpler "accept all cookies" option, the user has already demonstrated that they have concerns in this area, and are likely to be very accepting of UI sequences that make it easier for them to function within that choice with a minimum of confusion or
risk of not noticing new initial cookie decisions for a site.
Sorry about any formatting nasties in this response -- I copied most of it in from a text-based e-mail.