In context: Mozilla often tries to persuade users about its stance against intrusive advertising, promoting the Firefox browser as the best way to enjoy the web almost ad-free. Still, the San Francisco-based corporation is well known for forcing its own ads on users every now and then.

Mozilla officially promotes ad blockers on Firefox as a "best friend" for users forced to watch too many unsolicited advertisements while browsing the web. The average person sees "an average of 4,000 ads a day," Mozilla says, and that's a bit too much. And yet, Mozilla recently joined the crowd of ad-serving companies with an "unskippable" message trying to promote the company's own paid VPN service.

Mozilla VPN is a subscription-based product designed to shield users' activity online, with hundreds of servers in over 30 different countries to enhance privacy and the overall internet experience. The service is one of Mozilla's many ventures to seek additional revenue sources, as the company is still largely relying on Google Search money to fund Firefox promotion and development.

This past week, Mozilla started to show fullscreen advertising about its VPN service to disgruntled Firefox users. The ad campaign was met with universal displeasure, with many lamenting the "disruptive, intrusive" violation that goes against Firefox's supposed core values. The ad was shown through the browser's Messaging System, so it was difficult to block with a traditional ad-blocker like uBlock Origin.

Furthermore, the VPN ad was seemingly breaking Firefox and web browsing for several users. Many bug reports submitted to the Bugzilla platform were initially marked as "resolved" by Mozilla developers, as the ad was working as intended and there was nothing to fix. Power users could disable the VPN advertising altogether by changing the browser.vpn_promo.enabled config (on the hidden about:config page) to false.

In the end, Mozilla decided to stop the advertising campaign while apologizing to its users. The company now says it was just trying to "understand the best ways to communicate with people who use Firefox," but it accomplished the exact opposite of the intended result. The unwanted, intrusive "experiment" was quickly rolled back, as Mozilla already did several times in the past.

Besides being annoying and counter-productive, the VPN advertising fiasco was also exacerbated by a bug in the Firefox code. The browser's Messaging system is designed to show a Firefox-made promotional ad when user interaction has been silent for 20 minutes, but the VPN ad was seemingly being triggered inappropriately due to a time miscalculation error.