Tab Center started development over a year ago, and the WebExtension APIs that are the future of add-ons hadn’t been created at that point, so we chose to base it on older technology, knowing that it would probably become obsolete sooner, but also knowing that it was an short-term experiment, and that this implementation was never intended to land in Firefox. I mention this as an example to illustrate that many of the early Test Pilot experiments weren’t developed with 57+ in mind, and it wouldn’t have made much sense to do so. The newer experiments (as far as I know) are being developed in a way that will let them continue working far into the future.
Last year, I didn’t expect that there would be a lot of issues with Firefox 55 or 56, but since we started working on the Photon UI refresh, we have been landing a lot of changes into Nightly, because it’s a bigger job than just one release, and it is those changes that have been breaking Tab Center. My understanding is that most of them won’t make it to Beta until 57, so we may enable support for 55, or even 56 when those reach Beta, depending on how well it works at that time. I don’t see it as a matter of changing the goalposts, as much as the product going through some massive improvements, and Tab Center not being able to keep up.
I truly believe that, painful as it is (also for me!), these breakages are indications of great things coming to Firefox, and I wouldn’t be concerned. I’d also like to take this opportunity to thank all of you for your support and suggestions. Tab Center is one of the highlights of my time on the Firefox team, and you all are part of the reason why. 