Microsoft finally gets around to fixing half-decade-old Firefox CPU bug

Firefox logo on a macOS taskbar next to photos and Apple music
(Image credit: Shutterstock / Foxartbox)

Better late than never, I guess, but Microsoft finally got around to fixing a five-year-old high-CPU usage bug in Mozilla Firefox.

The bug, which is tied to Windows Defender's Antimalware Service Executable process, has been known to produce high-CPU usage when running Firefox compared to Google Chrome and Microsoft Edge. Fortunately, the issue looks like it is resolved at long last.

"According to Microsoft, this will be deployed to all users as part of regular definition updates, which are packaged independently from OS updates," Yannis Juglaret, a Mozilla developer, wrote on Mozilla's Bugzilla message board last month. "This includes even Windows 7 and 8.1 users, even though these platforms should not have had the performance issue with Firefox in the first place because the ETW events that cause it do not exist on these older versions of Windows. So as far as I understand, only users that would explicitly reject definition updates (which does not sound like something reasonable to do with your AV) would not get the fix."

That update has now rolled out, so Firefox users should hopefully see noticeably better performance.

Ok, so why did it take this long to fix?

Five years is a very long time for a bug fix.

And while it might be tempting to get conspiratorial and assume that not fixing a Mozilla Firefox bug is Microsoft's way of trying to get users to switch to Microsoft's own Edge web browser, it likely has a lot more to do with the issue being so limited in scope.

Firefox is a great web browser, but it's hardly the most popular. According to StatCounter's global Browser Market Share data, Firefox is used by just 2.93% of all users, while Chrome and Edge, which are based on the same Chromium foundation, account for just shy of 70% of the web browser market (with Edge making up a mere 4.64% of that total).

So, really, Microsoft probably felt it had a number of better things to do with its developer's time than to go track down a niche performance bug affecting so few users. And, according to Neowin, Mozilla's own developers appear to have been integral to getting the bug fixed, so it's likely that Mozilla had to do most if not all of the heavy lifting here.

John Loeffler
Components Editor

John (He/Him) is the Components Editor here at TechRadar and he is also a programmer, gamer, activist, and Brooklyn College alum currently living in Brooklyn, NY. 

Named by the CTA as a CES 2020 Media Trailblazer for his science and technology reporting, John specializes in all areas of computer science, including industry news, hardware reviews, PC gaming, as well as general science writing and the social impact of the tech industry.

You can find him online on Threads @johnloeffler.

Currently playing: Baldur's Gate 3 (just like everyone else).