URL fragment opening broken on Firefox on mystmd.org

[akhmerov]

Description

I observe on Ubuntu 25.10 + Firefox 145.0.1 that opening a URL on https://mystmd.org/ with a URL fragment sometimes fails to scroll to the fragment, presumably because of how the page loading and rendering works.

Oddly, this seems to only happen to some URLs, like e.g. https://mystmd.org/guide/deployment#deploy-base-url

I also see that the problem doesn't occur on chrome.

To reproduce:

1. Navigate to https://mystmd.org/guide/deployment#deploy-base-url
2. Observe that the top of the page is shown
3. Go to the browser address bar, click enter. Observe that the page scrolls to the correct anchor.