I've found it hard to skim diffs on hgweb (example). The lack of visual hierarchy meant I often didn't notice the jumps between files, even when I was looking for them.
I wrote a user stylesheet to fix this. Between files, it adds a gray bar, vertical space, and outdented filenames. Between sections of a file, it adds a small amount of vertical space. (screenshot before, screenshot after)
To install hgweb-visual-hierarchy.css, add it to chrome/userContent.css in your Firefox profile directory. You may need to create the subdirectory and file.