People can run whatever software they want. They don't need your permission, or to know how to file a bug report, or whatever other bullshit you've dreamed up. They're also allowed to complain when a site is broken in their browser.
If you don't want to see unsophisticated complaints and bug reports, then you need to stop looking at them. Perhaps find a different profession/hobby where your user-hostile attitude actually benefits someone.
It’s also simply not true that more user-facing bugs happen on nightlies. I run nightlies at times and have never found them to be less stable in any tangible way that would affect site usability and show up in a bug report. On the contrary, nightlies are the first to get important bug fixes. It’s overwhelmingly likely that GP is just a lazy audacious dickwad ignoring valid bug reports.
To some extent that is correct, ironically this was likely an attempt to reduce user complaints and bug reports. The problem is that they didn’t went about the the correct way there is a better way to implement feature detection than to check the user agent: https://developer.mozilla.org/en-US/docs/Learn/Tools_and_tes...
If you don't want to see unsophisticated complaints and bug reports, then you need to stop looking at them. Perhaps find a different profession/hobby where your user-hostile attitude actually benefits someone.