[Edit: This was not the solution. It worked the first time, but now it isn't working again.]
I was using Citrix Receiver quite successfully on Ubuntu 16.04 and Linux Mint 13 to remotely access my customer's network, but I couldn't make it start in a window. It was coming up in full screen mode. I could minimize the whole Citrix session by doing Ctrl-F2 (to tell Receiver to pass the next key to Linux), then Ctrl-Super-downarrow (Super is the "Windows" key). However, I wanted to be able to watch the Citrix session on one monitor, while I worked on other stuff on the other monitor.
I finally found this blog that told me how to set up the Receiver config files to get Receiver to start in a window: http://blog.eek-a-geek.info/2014/10/citrix-receiver-for-linux-131-on-64-bit.html. What it says is:
Edit "~/.ICAClient/All_Regions.ini", replacing the line "TWIMode=*" to "TWIMode=Off".
Edit "~/.ICAClient/wfclient.ini", adding a line "TWIMode=off" to the "[WFClient]" section, and adding a line "UseFullScreen=True" to the "[Thinwire3.0]" section.