There are tools available to disconnect those connectors, but in my experience, none of them worked effectively for me. Folks on this Forum have recommended to first push in the connector and rotate it slightly. Then, while pushing in the connector, push in the two tabs on the connector and pull them apart, while holding the tabs pushed in.
If the above fails, then you can take a pair of pliers and break the connecting plastic piece on the connector, just below the tabs.
IMPORTANT: Wear a pair of gloves when working on the Tee connectors, as the insulation on the firewall is quite aggressive, and you will be itching for the next few days.
Also, be sure to replace ALL heating and cooling rubber hoses as part of this project, as you will probably find the original manufacture year stamped on the hoses.