I know it doesn't break EVERY customisation available, because I used it the Australis Nightly with addons. But in regards to the theme... and? Anytime ANY piece of software that allows themes makes ANY changes it has the possibility to break existing themes. That's been known since forever, and Firefox is no different.
Originally Posted by JX8p
If the theme is unmaintained then you had to know that eventually it would stop working, well now's that time.
If the theme IS maintained then shame on the developer of that theme for not making an Australis version. This wasn't a big 'shock' change, this has been known about MONTHS and has been able to be played around with since last year. Hell, they even made special versions of the nightlies available last year that were current-nightly + australis so that those who were interested, and developers, could start playing around with it and figuring it out.