Best way to virtualize Thunderbird/mail client

I'd like to isolate my mail client (Thunderbird) from the rest of my system.

I installed virt-manager and made a virtual machine, installed Ubuntu and Thunderbird.

Then I used X forwarding over SSH on localhost to launch Thunderbird on my desktop, while it runs on the virtual machine.

This works fine except the UI is extremely slow and sluggish. It tends to lock up and crash. I have SSH compression turned on with little effect on performance.

I know I can run Thunderbird with firejail, but I really don't want it to have access to anything on the host OS.

Is there a faster, more performant way to virtualize an X application?

Thanks