Show the number of Hacker News comments for current tab.
When Hacker News links to the page you're viewing, the toolbar button will display a badge with the number of comments. Click the button to open the comments in a new tab. When the page hasn't been posted on HN, the button will open the "submit" page with url/title pre-filled.
One caveat is that the badge isn't updated in real time, only when the URL changes.
If there are multiple linking pages on Hacker News, it will pick up the one with the most comments.
The URL fragment (the part after #) is stripped when looking for the referencing HN page.
Source code: https://github.com/ivan7237d/hn-comments