Update: as of now (1:10pm PT), it looks like all major features are back to normal.

Due to a recent server migration, AMO has been experiencing some problems for the past few hours. While the site is up, installing and submitting add-ons isn’t working correctly at the moment. The developer and operations team are working hard on fixing these problems as soon as possible and we will update this post once the problems are resolved.

https://blog.mozilla.org/addons/2015/11/11/amo-experiencing-technical-difficulties/