Starting with Firefox 65, form elements like
<select> will allow to dispatch standard and custom events even when they are disabled. This change aims at improving the interoperability among browsers and responding to web developer needs.
Given that the element is disabled and unfocusable, most of events such as
input won’t be fired by regular user interactions, but
mouseleave will now be fired unlike the current version of Google Chrome, Apple Safari and Microsoft Edge.
In order to avoid any unexpected behaviour, it might be better to always check the
disabled property on the form element before further processing within your event handler.
Update: CSS animation and transition events will also be fired on disabled elements on Firefox 67 and later.