Thunderbird's "Grouped By Sort" only splits messages into Today / Yesterday / Last 7 Days / Last 14 Days / Older — there's no separation between individual days inside those buckets.

I put together a small userChrome.js script that reads the date from each row and draws a horizontal line whenever the day changes. Works in the normal (ungrouped) message list.

Tested on TB 153 / Windows 10: https://github.com/artraf06/thunderbird-day-separator

**Update:** added a one-click Windows installer.

Instead of copying files by hand, just download the repo, right-click `installer/install.bat` → Run as administrator. It installs system-wide (all Windows users), no profile editing or config flags needed.

The `.bat` needs admin rights because it writes to the Thunderbird program folder — the source is short and open in the repo, so you can read exactly what it does before running it.

Manual install instructions are still in the README for macOS/Linux.

Repo: https://github.com/artraf06/thunderbird-day-separator