@floridamatt Everybody surely agrees with you that add-ons regressing with new versions of Firefox is a problem; this is a major flaw of the XUL model which makes it very challenging to make changes to Firefox without breaking add-ons that have modified the UI.
This is one of the important benefits of WebExtensions: rather than have add-ons prescriptively telling Firefox how to change the browser, it declaratively says what it wants to do, and it's up to Firefox to determine how best to implement that. WebExtensions will let us be more agile with UI changes without risking add-on breakage. It's going to be great.
That said, those add-ons breaking is an orthogonal problem to Tab Center. Both Tab Mix Plus and Tree-Style Tabs add incredibly useful features to how tabs work in Firefox, but those improvements do come at a cost (cognitive load, interface complexity, etc).
Tab Center was looking to test a simpler, more streamlined side tabs interface suitable for mainline audience. Will it get merged into Firefox exactly as it is? That doesn't seem particularly likely (@bwinton may have more to say about that), but it also served another purpose: helping us understand what sorts of WebExtension APIs we need to build to enable that sort of experience in the new add-on format to ensure that add-ons like Tree-Style Tabs and Tab Mix Plus are able to live far into the future. That has certainly been achieved.