In chrome if you have a closed details tag and your url anchor targets the ID of its summary element, the details tag will be opened (on page load / hashchange). It would be nice to see this behaviour in FF as well.
Using: test.html#test
<details>
<summary id="test">Test open by ID</summary>
<div>
I will be open
</div></details>
<details>
<summary>Test closed</summary>
<div>
I will be closed
</div></details>