I open a lot of tabs and keep my browser open for days. After a while, Firefox becomes slow, while Chromium always feels fast. It's hard to measure, but it's real. In addition, sometimes I encounter sites that do not work or are slower in Firefox (I don't have an example in my mind right now).
Also, more than once, I see that something is not supported/slower in Firefox just because of some unresolved bugs. They get fixed after a while, but that makes Chrome always ahead and ready when you need a specific feature right away. See for example: