Issue is introduced in yesterday's push.

Example: https://reviewers.addons.mozilla.org/editors/review/545420/
Compare: https://reviewers.addons.mozilla.org/en-US/firefox/files/compare/974497...965277/

All links on the left side panel load a /browse/ version without the diffs instead of /compare/
e.g.: https://reviewers.addons.mozilla.org/en-US/firefox/files/browse/974497/file/install.rdf

<li> <a class=" file selected diff" data-delay="0" href="/en-US/firefox/files/browse/974497/file/install.rdf" data-short="install.rdf"><span>install.rdf</span></a> </li> 

Let me know if more examples are needed.