I have Firefox set to prompt me to accept cookies, mainly because I don’t like the tracker cookies, and set most cookies to be session-only or denied, depending on how annoying the website is. You get a good sense of how frequently sites set cookies this way, and can see that most of them are advertising-related, and provide little to no additional functionality to my use of the site. Some of them are to store credentials, or preferences, and those are tolerable. From a design standpoint what bothers me is that a lot of sites seem to set cookies as part of establishing a session, when there is no need for a session in that context.
But what really bothers me is that when I open a tab in the background, Firefox switches context to that tab in order that I might confirm the cookie, and I then lose my train of thought. Stop it.
This one from the Dept. of State has a funny name: ForeseeLoyalty_MID_IJ80MItY0t
One method of fixing this problem, if the operating system does not let you attach the message box to the tab without bringing the window to the front, is to do as Google Chrome does and open the new tab immediately adjacent to the previous tab, rather than at the end of a long tail of tabs.