Pseudalopex pretty much nailed it; Google has strongly resisted adding sidebar APIs to Chrome, or allowing add-ons to hide / relocate the tab bar. Those are two

Pseudalopex pretty much nailed it; Google has strongly resisted adding sidebar APIs to Chrome, or allowing add-ons to hide / relocate the tab bar. Those are two of the major reasons why it's difficult to build a quality Tree Style Tabs add-on for Chrome. Firefox will explicitly include APIs that allow that, which should be landing soon. Most of these changes and their timelines are motivated by fundamental architectural work happening deep down in Firefox. We're doing the best we can to get as many add-on APIs in place before 57, but for heavy add-on users, it has the potential to be a rough transition. For deeper meddling in the browser, and for APIs that don't yet exist, WebExtension Experiments (https://webextensions-experiments.readthedocs.io/en/latest/index.html https://webextensions-experiments.readthedocs.io/en/latest/i...) should provide a reasonable option. Basically, you can keep using old-style "bootstrapped" add-ons in DevEdition and Nightly. Kind of an explicit way to opt-out of API stability guarantess so we can continue refactoring the browser core. Experiments are intended for prototyping new WebExtension APIs for eventual inclusion into mainline Firefox, but you could also use them for power-user APIs, so long as you don't mind the maintenance burden.