That only one instance of Telegram can run at any time.

And if you open a Telegram link it will open in the existing instance.

Windows uri handlers actually always create a new process, though; so if you want this single instance behavior, you have to do some check at the start of the process and communicate the uri to the previously running process (as explained in the article).

Because I do know what this vulnerability was about, and it had nothing to do with Firefox (and I know that you specifically probably didn't mean that Firefox was involved, but even Telegram's desire to be single instance was not what caused the vulnerability).