Fair question. I use HN Replies, which emails me when people reply to me. On my computer, if I click the link in the email, it opens in Chrome. But on my computer my normal browser for HN is Safari. I use separate browsers for different things on my computer, so Safari is for HN and reddit, Chrome is for Google apps, like gmail etc., and Firefox for everything else. So sometimes I also get into HN via Firefox if I'm there an happen to type the URL or get a link.

Then on the phone, Safari is again the default browser, but if I click a link from the gmail app it opens in mobile Chrome.

So to use a plugin to change anything, I'd have to have the plugin on all five browsers.