I don't think Firefox on my pc should just open the website I sent into a tab, it should ask me to open it or have a setting to either ask or to just open.
Firefox for Android does this, when I send a tab from my desktop to my phone, I get a notification on my phone about the tab and can choose to ignore it or open it.
I could see where someone uses a bug or malware to force your phone to send a tab to other devices to spread malware.