Let's say that Firefox is your default browser and that you manage your mail with an email client. When you click on a URL link in an email message, FF will open up the corresponding page in a new tab. If you click again on the same link, FF will open up that same page in yet another new tab, and so on. This doesn't seem too inconvenient. But if you set your default browser to Safari (on Mac or iOS of course), it appears that the browser first checks to see if a page for that same URL is already open. And if it is, it will simply reload that page in its tab and bring that tab into focus in the browser window. This behavior isn't limited email clients. It appears to apply to any app that uses weblinks (and doesn't have its own browser integrated into it). It might be that when Safari receives a URL from another app it overrides target="_blank" by inserting a target name unique to the URL – perhaps a hash of the URL. That way, each subsequent time the browser receives a request for the URL, it will load it into its appropriate tab. From the user standpoint, this is a really nice feature, but it's also a plus for app developers to better streamline how their apps might work with browsers. It works so well in Safari I'm hoping that we'll see it soon in Firefox.
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