Asklemmy

I've never been able to confirm if it's true or not but around 2014/2015, I had a malicious Firefox extension that apparently originated from Google Chrome. What it did was basically put ads on all webpages, including blank pages and it was really hard to remove because it would just keep reinstalling itself until I uninstalled Chrome and then found and deleted the folder that contained the origin of the malware.

I wasn't able to do much research on my own, mostly because I didn't really know how to, but everyone online (possibly including Mozilla themselves) who was infected by the malware believed that Chrome downloaded the malicious Firefox extension. The main reason people believed it was because not only did the malware only seem to infect users who had both Chrome and Firefox installed but the origin of the malware would keep reinstalling itself until you removed either Chrome or Firefox and stuck with just one browser.