Are the all browser in different operating system writen in same code/tags/elements in same way ?

Question

Suppose to say
Firefox browser written for window ,unix or Linux are same for all the three OS or is there any changes made as per the OS for issue of executing.

Answer 1

No, I believe they operate the same.

Answer 2

No, that should not be different. But javascript engine must be different from browser to browser.

Answer 3

Nooo