Unfortunately you can't. The desktop and touch versions now share the same profile and we can't have both accessing it at the same time.

The desktop and "Windows 8 Touch" (modern) app now share the same profile. This means you now have access to the same bookmarks, history, saved passwords, etc. from both apps. However, because of a limitation of our profile system, you can't run both at the same time while they are sharing a profile.

However, you can run both at once if you first create a separate profile for each app:

1. Start the profile manager, create a profile named "desktop", then exit the Profile Manager.
2. Create a shortcut to launch Firefox with these command line arguments: -P desktop -no-remote
3. Use the shortcut to launch Firefox using your "desktop" profile.
4. Use the normal Firefox icon to launch a second copy of Firefox using your default profile.
5. Set up Firefox Sync if you want the two profiles to have the same bookmarks and other data. (Treat the two profiles as two different "computers" while following the setup instructions.)
6. In the default profile, use the "Relaunch in Windows 8 Touch" menu item to switch to the modern app.

I'm sorry this process is so complicated! We want to make it easier to use both applications at once, but we may need to make some very complicated changes to our code to make that possible, so it's not likely to happen quickly.

For more tips and documentation on working with profiles, see this community article about Using multiple profiles.

Matt's solution worked very well. It might be helpful to have this information available somewhere with the other details about what's new in Aurora. I'm sure I'm not the only person who likes using both modes simultaneously.

One caveat - I had already set up Aurora with the old Firefox Sync service, and after unlinking and setting up the new Sync, I had to restart Firefox a few times with both profiles before everything started working as Matt described in his post.

Thanks!

Update - this initially worked, but after rebooting my computer, it's not behaving anymore. If I start Aurora from the desktop using my shortcut and then launch it from the Start screen, the Start screen tile goes back to the open desktop instance. Is there something else I need to do?

You might need to create a second shortcut with "-no-remote" to launch a second desktop Firefox in your default profile, and then switch to Windows 8 Touch from that desktop Firefox window.

These steps are documented on our developer wiki page, and I'll see if I can make them easier for users to find too. As you can see, we haven't got them quite nailed down yet.  :)