Unfortunately container tabs are not yet supported for Android Firefox. FoxyProxy is by no means malicious, it certainly has good intentions and for the most part it does what it says it does. The problem is that due to many fundamental design choices it is too easy for information to leak. Plus the URL regex matching can be very finicky to work with. I used FoxyProxy for years but gave up on it after a while since I could never get a consistent and reliable experience out of it.
At this point I just alter the proxy settings in Android ff directly - no extension. Unfortunately ff profiles are also not supported on Android yet so if you don't want your main browser to always be attached to a proxy you can install other releases of ff such as Nightly and just configure that with proxy settings. If I am recalling correctly, you can also configure ff to apply proxy to either normal or private browsing too.