My previous tests (above) were with that on. I also tested with it on in Finder (like, when you set the file viewer to tree mode). There, when you VO navigate through the tree items, selection follows. So your keyboard commands are always "synced" with your VO commands. With it off, I get the same behaviour.
Bug 1723777 Comment 3 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
My previous tests (above) were with that on. I also tested with it on in Finder (like, when you set the file viewer to tree mode). There, when you VO navigate through the tree items, selection follows. So your keyboard commands are always "synced" with your VO commands. With it off, I get the same behaviour in FF, and finder matches FF behaviour (so, selection doesn't follow VO unless you manually activate an item with VO+space. keyboard commands go to the selected item)