iCloud stopped working for me again. I point the browser to the main iCloud page, and the little wheel spins for a while, then a message box comes up, offering to send the log to Apple.
The web console shows the following message, then it shows an ad to come work for Apple as a developer, and then a lot of other error messages.
mutating the [[Prototype]] of an object will cause your code to run very slowly; instead create the object with the correct initial [[Prototype]] value using Object.create
iCloud has been working for me since the release of Firefox 38 beta6. Before that it crashed with a different error, and before that it was working on Firefox 31. I imagine that Apple has now made changes that Firefox 38 beta 7 cannot support.
To see the error, point the browser at
https://www.icloud.com. I'm hoping a newer port of Firefox, or else a bug fix will restore access.