In Firefox, the
window.find method could be used to open up a dialog that let the user find strings on the current page. It turns out that the dialog has been broken like
window.showModalDialog since the multi-process mode codenamed e10s was enabled with Firefox 48. Starting with Firefox 53, the dialog is broken even when e10s is disabled in the browser.
Given that no other browsers open the Find in Page dialog, Mozilla developers have decided to explicitly remove the support for the
aShowDialog argument rather than restoring the functionality.
There’s also a longstanding plan to remove the non-standard method entirely, so it’s discouraged to use it anyway.
aShowDialog argument support has been removed with Firefox 53.0.2.