It's an extension, not a browser, but Tree Style Tab includes this functionality plus a few additional features that I've found helpful:
The "+" button at the bottom of the tabs list has a sub-menu that allows you to open a new tab in several different locations:

In settings, you can choose which of these options is the default:

And you can set custom keyboard shortcuts for each (note that these are my own custom shortcuts; I don't recall what the defaults are):

This level of complexity might be overkill for Orion, but I'd love to have the ability to open a new child tab using a keyboard shortcut in addition to the adjacent (sibling) tab behavior discussed above.