In May, Mozilla engineer Dan Witte proposed a mechanism that caused cookies to automatically expire when a user closed his or her Web browser. (By comparison, most tracking cookies last for years). It only affected tracking cookies—not cookies that websites use to remember users' passwords or shopping cart information.Mr. Witte's proposal was inserted into a developers' version of the Firefox browser on May 28. By early June, however, the news trickled out to advertising industry executive Simeon Simeonov.Mr. Simeonov is the co-founder of a company, Better Advertising, that provides technology to online-ad companies. When he heard about the change, Mr. Simeonov said he worried it "would have broad, unforeseen impact on the consumer experience and perhaps even on the Web ecosystem."Mr. Simeonov reached out to the chief executive of Mozilla, who put him in touch with Jay Sullivan, vice president of products at Mozilla. The two spoke on June 9. Mr. Sullivan said Mr. Simeonov expressed concern that the change would prompt advertisers to "go underground" to conduct even more surreptitious forms of tracking. Mr. Sullivan said that Mr. Simeonov's comments "supported what we were already thinking."