Hi 
You can cheat a bit xD
Try making a script that will take a argument and open a link in a new tab, like this :
#!/bin/bash
firefox -new-tab $1
Next create a default entry :
sudo update-alternatives --install /usr/bin/x-www-browser x-www-browser /opt/fake-browser 10
Next choose it as default browser :
sudo update-alternatives --config x-www-browser
Give it a try running twice this commands :
sensible-browser google.com
sensible-browser google.com
sensible-browser command will run your default browser.
I was not able to reproduce your issue exactly as you have it, but I think this could work. I hope will work for you