This script disables the third argument (features) of the Window#open() method, and deletes the Window#showModalDialog() method.
I wrote the script because, in past versions of Firefox, browser.link.open_newwindow.restriction;0 had been ignored on user.js / prefs.js / about:config if dialog=yes was passed the third parameter (features) of the Window#open() method, or the Window#showModalDialog() method was called. However, dialog=yes was obsoleted in Firefox 44, and the Window#showModalDialog() method did too in Firefox 56, so the script is unnecessary in Firefox if you set browser.link.open_newwindow.restriction to 0.
* On Microsoft Edge extension Tampermonkey, user scripts only work in pages that Content Security Policy allows eval() function because of Issue #11320212. If you open a page that eval() function disallowed, extension blocked is reported automatically. The script is affected by the bug of the browser because the script applies to all web pages.