Firefox has a rapid release cycle - every 5 or 6 weeks. Unfortunately, for people who support faculty desktop machines and student lab machines/environments, keeping the browsers updated to keep within the latest 3 releases for Canvas support becomes a large task.
However, Firefox offers an "Extended Support Release" - this release does not add new features, just security updates on a regular basis. If Canvas were to include the ESR versions as supported browsers, it would make managing lab and faculty machines much easier for IT departments everywhere.
Comment from Scottd at Canvas on May 28 in the comments below:
Thank you to everyone who took the time to carefully read through and comment on this feature idea (in the current forum and, many of you, in the old forum previously). This is obviously an important issue for a lot of Canvas users, particularly in large computer lab deployments. After weighing all the opinions expressed, we have decided to continue with our decision not to support the Firefox ESR browser.
Considerations that supported this decision:
Relatively few of the people logging into Canvas currently use the FF ESR browser (less than one percent by recent count).
Because Firefox is relatively stable when compared to other popular browsers, users of ESR rarely encounter errors that would be alleviated if we quality checked every change to Canvas against this browser.
Not releasing features that take advantage of newer web technologies because they would not run reliably in a year old browser would necessitate delaying the release of new features to Canvas users who do not use ESR
Several comments in the thread make reference to the alternate idea of making the unsupported browser warning message configurable or less intrusive as a possible compromise, which would be another viable feature idea. If you like this idea, please consider voting for Dismiss the Unsupported Browser warning during a browsing session, starting June 3rd.