I develop a Firefox add-on. In our experience, the transition to Australis was pretty smooth. Our add-on bar icon was automatically moved to the toolbar without any changes on our part, although we ended up changing the icon to better match the Australis UI. The transition might not be perfect for all methods of overlaying the add-on bar, but I think the folks at Mozilla are doing the best they can.
I don't think Mozilla should be holding back functionality to avoid breaking extensions. If an add-on is actively maintained, then the developer will update it, and if the add-on is not actively maintained, then it's going to have to break someday.
I don't think Mozilla should be holding back functionality to avoid breaking extensions.
I respectfully disagree. Extensions are the USP for Firefox, and IMHO breaking compatibility should be considered a serious issue and not something to be done lightly. (See also: all the hate when they switched to doing six-weekly updates and it kept disabling extensions every time for months afterwards.)
In any case, there was no pressing need to remove the add-on/status bar. Arbitrary rearrangement of UIs is generally bad for usability. Ditto for major things like moving tabs (for anyone who isn't using an enhanced tab plug-in anyway) and the burger menu (which is now a tiny area right underneath the close window button, and appears to be completely inaccessible by keyboard).
I don't think Mozilla should be holding back functionality to avoid breaking extensions. If an add-on is actively maintained, then the developer will update it, and if the add-on is not actively maintained, then it's going to have to break someday.