You did not say much about your methodology, so just general things that I imagine played into this: - What is a "user"? Is it someone who downloaded it once?

You did not say much about your methodology, so just general things that I imagine played into this: - What is a "user"? Is it someone who downloaded it once? Is it just straight up the number of installs that ever happened? Or is it someone who actively uses the extension on a frequent basis? If you go by the numbers on the Chrome Store and on AMO, they likely have different definitions. - Firefox recently introduced support for an extension API which is quite similar to the Chrome extension API (for the most part a superset), so lots of Chrome extensions have been ported over from Chrome just as recently and as a result will naturally not yet have as high user numbers. - Firefox has traditionally had more powerful extensions and still continues to have a more powerful extension API. So, there's going to be extensions that are better, which only exist on Firefox, whereas similar extensions that were ported over from Chrome, therefore exist on both, will largely be ignored. Analysing this on a non-empirical level: - Firefox has less users. - Chrome has somewhat less functionality built-in, which means users will have a higher need for extensions. - With Firefox's extensions having traditionally been more powerful, the user base should still have many tinkerers. - Firefox extensions are much more thoroughly vetted and malware is essentially a non-issue. On Chrome, it happens more often and I have seen people removing non-essential extensions for that reason. - Chrome users are less likely to be privacy-conscious, which somewhat negates the previous point again and means they're more likely to just install an extension without thinking about it too much. On the other hand, Firefox users should be more likely to install privacy-enhancing extensions. - Firefox for Android supports many extensions as well (though its user base is not the biggest).