The Firefox DevTools console generally groups output.
For example, executing the code of your screenshot shows this:

Though, when you output objects it may happen that they are not grouped. If that's the case, check if the data is really identical (incl. source links). It may be that your output refers to different objects. E.g.
for(let i=0;i<100;i++) { console.log(["test"]); }
will output 100 times Array [ "test" ]
.
If you're sure your output refers to identical objects, ensure you have the latest version of Firefox installed. If it still occurs in the current version of Firefox, you should file a bug report.