I'm connected right now through a Firebox 500. My Mitel 5224 handset connects directly, and Your Assistant connects through the Watchguard Mobile VPN.
I don't have YA set to "Teleworker Mode", but I get the presence information for other users, chat windows, and the pop-up windows for transferring calls, etc.
I'm not even remotely an expert on the Firebox. I just openned the ports that one of the Mitel documents called for - either a YA or a Teleworker handbook.
If this is what you're looking for, I can take a closer look at the setup.