It appears that sometime around April 22nd, 2019, Instructure made what appears to be an unannounced change that resulted in a "Unsupported Browser" warning message appearing on Firefox ESR releases.
After doing a bit of digging, it looks like Instructure is opposed to supporting the ESR release Include Firefox ESR as Supported Browser
My main question is why? The ESR (Extended Service Release) is commonly deployed in large organization and education institutions. Canvas being a LMS (Learning Management System) that is heavily used in these environments this decision does seem odd. However, we have heard several Instructure employees make comments that Google Chrome is the recommended browser. Maybe this is some weird way to push users away from Firefox?