Is there a good short answer for why uBlock is using constructs like this: if ( self.browser instanceof Object ) { self.chrome = self.browser; } else { self.browser = self.chrome; } Instead of using a polyfill like that one https://github.com/mozilla/webextension-polyfill ?
This may sound shocking to "modern Javascript" developers: Often, a little copy is better than a little dependency. More often, a little copy is better than a large dependency.
Using a full well-written* dependency for something not part of your "core competency" (even something seemingly trivial) is often a better choice in my opinion. That dependency will often know the "unknown unknowns" to you since their "whole purpose" is solving that one issue.
In this case, and from what I can see from my VERY small glance at the code, I'd argue using a polyfill would be a better choice in this situation. The polyfill has cross-browser tests, ensures compatibility, continually tests against new browser versions and will alert you and shield you from things if they change, download size is less of a worry for extensions and most of it no-ops away in 100% supported browsers, and it's maintained by mozilla.
Obviously there might be additional reasons why the uBlock origin developer isn't using it, it might be super overkill for his needs, he might consider the browser API part of his "core competency" and therefore shouldn't be relying on 3rd party code for it. I'd trust him more than myself here since I know so little about the domain. And I had a similar question to the GP commenter of why he decided to avoid the polyfill.
Your little saying isn't a useful answer, and the dig at "modern javascript developers" is not only unnecessary but also a real annoyance for me because it's used to dismiss just about every opinion while providing no real reasoning why you think that way. If you think a little copy is better, explain why! Don't just put down a group and leave without giving out any useful information!
* A "well written" dependency meaning something well tested, has a solid development team or dedicated person, and has somewhat widespread usage. Obviously it will differ depending on your needs.