Ctrl-D makes it too easy to create bookmarks accidentally
It used to be the case that pressing Ctrl-D would pop up a dialog box prompting you to create a bookmark (or cancel.)
A few releases ago, Firefox changed this behavior. Now, Ctrl-D creates a bookmark, then pops up a dialog prompting you to edit the bookmark (or delete it.)
This is a subtle distinction, but potentially an important one, for two reasons.
1. Pressing Escape after Ctrl-D doesn't undo the bookmarking operation as you might expect. It's very easy to press Ctrl-D by mistake when you mean to press, say, Ctrl-F. If you've just pressed a key you didn't intend to press, without knowing what it does, and an unexpected dialog appears in your peripheral vision, it's natural to react by pressing Escape ("oops, didn't mean that.") And if you do that, and the dialog disappears in response, it's quite natural to assume that you successfully cancelled whatever action it was that you inadvertently initiated.
2. Pressing Ctrl-D immediately saves the current URL to disk (namely, in places.sqlite), without any further confirmation. Even if you are paying attention, a simple slip of the finger can potentially create a persistent record of your browsing activity. (Even if you delete the bookmark immediately, it won't be purged from places.sqlite right away.)
This UI change was a bad idea, but in "normal" Firefox usage, it's usually only a minor annoyance - I end up with a bunch of random accidental bookmarks at the bottom of the menu that I need to clean out every couple of months. But in the Tor Browser context, it's potentially quite dangerous, as it violates the disk avoidance principle.
Saving bookmarks without the user's consent may or may not have any practical impact in most cases. But it can have a major impact on users' confidence in the browser. For that reason, Tor Browser can and should do better.