The makers of the popular Firefox Web browser are exploring ways to create a do-not-track mechanism that could offer Internet users a way to avoid being monitored online.

The effort comes just months after Firefox’s creator, Mozilla Corp., killed a powerful and new tool to limit tracking under pressure from an ad-industry executive, The Wall Street Journal has learned. Mozilla says it didn’t scrap the tool because of pressure, but rather out of concern it would force advertisers to use even sneakier techniques and could slow down the performance of some websites.

Meanwhile, online advertising company Lotame Solutions Inc. is also supporting efforts for an industry-created do-not-track mechanism. Lotame’s powerful tracking technologies were featured in a front-page article in the Journal earlier this year.

The tensions reflect growing concern about the burgeoning trade in personal information online. Increasingly, advertisers don’t want to simply buy ads online—they want to buy access to specific people they consider most open to their message. The data-gathering industry is the subject of a Journal investigative report, “What They Know.”

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