We do not add just any old or new browser to webrowser's predefined list.  (And there are likely some that should be dropped.)  The last addition was the Chrome group in 3.3.  Neither Microsoft Explorer or Edge are listed.  I believe the former was considered to be covered sufficient well by 'windows-default'.

On Windows and Mac, one can use any browser by making it the default.  It is also possible for any browser to make itself easily usable from Python by adding a module that registers the browser controller class on import.  You should suggest this to the browsh dev and see if *they* care.

Looking further, there are about 10 browsers with measured usage listed in https://en.wikipedia.org/wiki/Web_browser that are not on the webbrowser list.  There are more at https://en.wikipedia.org/wiki/List_of_web_browsers, including  Browsh appears on neither list.  It does appear on https://en.wikipedia.org/wiki/Text-based_web_browser.  But it seems too obscure to deserve being added.  So I think this request should be rejected.