The nonprofit, best known for developing the Firefox browser, took out a full-page ad in Tuesday’s print edition of The Washington Post to ask Facebook CEO Mark Zuckerberg and Twitter CEO Jack Dorsey to “immediately” disable recommendations of Facebook groups and Twitter’s trending topics until January. The ad includes names of some of the 6,000 people who signed the letter.

“This is a critical step in preventing the viral spread of disinformation—such as inaccurate voting procedures or election results,” the ad said. “We urge you to do more to curb disinformation in order to protect the integrity of the U.S. election.”

Facebook and Twitter did not respond to requests for comment.

The ad is part of Mozilla’s efforts to fight misinformation online that began in 2019. Meanwhile, Facebook and Twitter are struggling to police the conspiracy theories, misinformation, and fake news posted by users, politicians, and even the Trump campaign during a hotly contested election. 

Before buying today’s newspaper ad, Mozilla had launched separate petitions against Facebook’s group recommendations and Twitter’s trending topics that in total garnered about 30,000 signatures in support. Mozilla also raised $80,000 from more than 4,500 people for its campaign.

Before the ad published, Mozilla had requested meetings with Facebook and Twitter to discuss the issues directly. Facebook took the meeting while Twitter didn’t.

“There definitely weren’t any firm commitments at the meeting,” Ashley Boyd, Mozilla’s vice president of advocacy, said about the meeting with a small group of Facebook staffers representing product, policy, and communications.

Mozilla first became active in trying to curb misinformation last year, after it signed the European Union’s Code of Practice, a voluntary commitment by big tech companies to fight disinformation, be more transparent about political ads, and close fake accounts. Since then, Mozilla has been tracking whether the signatories have followed through on their promises. 

Mozilla has since called out Facebook and Google for allowing ads with disinformation during the United Kingdom’s recent elections. It has also publicly urged Facebook to provide more transparency within its ad library, a searchable archive of all the ads on its service, and joined the Stop Hate for Profit campaign, which encouraged companies to boycott Facebook ads in an effort to pressure the company into better policing hate speech on its service. 

Tuesday’s ad is intended to encourage Facebook and Twitter to address “systemic changes” that will do more than address issues case-by-case. Sinan Aral, a researcher at MIT who is focused on social media, said public pressure is one part of the solution to fixing the issues that currently plague social media. 

“In general, that whole social momentum is important,” he said. “That’s going to mobilize users, legislation, company policy changes—it all works in concert.”