On July 11, 2016 GitHub changed their font ordering to prefer Segoe UI on Windows. I think the old ordering which preferred Arial over Segoe was better. I wrote this CSS for Stylish to revert some of the most prominent changes.
In FF install by Stylish > Manage Styles > User Styles > Install from URLs...
https://rawgit.com/jay/e99fe4fe7e7bd4feaf5f237e15717133/raw/GitHub-Ignore-Segoe-UI.css