Firefox, despite the impression that HN might give, is still very configurable. The existence of about:config means Mozilla can add options without cluttering the more accessible preferences menu. I'm perfectly happy with the default behavior of selecting everything by default, but it does seem like the sort of thing that would be nice to make configurable by those who are willing to mess with about:config.

The linked bug report is literally that one of the about:config preferences has been removed and will no longer be respected.

It adds a lot of complexity and therefore cost unless you don't care about testing.