Starting with Firefox 42, the new multi-process mode called Electrolysis (e10s) is enabled by default on the Firefox Developer Edition (Aurora) in addition to Firefox Nightly. Since e10s involves numerous drastic changes to the Firefox core, many Web features are still not working properly. See the dependency tree of Bug 516752 for the open issues, and please file a bug if you have encountered any other problem. Note that some issues may occur when e10s is disabled, like Bug 1188818.
Update: This change has been postponed to Firefox 43 due to the “less coverage of non-e10s testing”.