Sure! First, I want to clear up three related terms that have been confused for each other in a number of bugs: tip results, Search Tips, and Interventions.
Tip results are a new result type in the Urlbar. Tip results are three times the height of a normal result and contain an icon, some text offering a "tip" based on the context the user is in, a button with which the user can act upon the tip, and an optional help/"what is this?" button. We checked the a11y of the tip result in bug 1578445.
Interventions are one of two new applications of tip results. They are tip results that appear in response to specific searches. For example, if a user searches "clear history firefox", we will show a tip result ("an Intervention") that offers a button to clear history. We have another that appears in response to a search for "firefox is slow" that offers a button to refresh the user's profile. There is an open bug to check Intervention's a11y at bug 1606921.
Search Tips are the other application of tip results. They appear when the user browses to a new tab or to the homepage of their default search engine (e.g. google.com). The text of the tip encourages the user to use the Urlbar to search instead of google.com. The actionable button reads "Okay, Got It" and focuses the Urlbar. We only show Search Tips a few times over the lifetime of a profile and only under certain conditions. In the new tab case, we open the view and it contains a single tip result ("the Search Tip"). The Urlbar is focused, as it usually is when a new tab is opened. In the search engine homepage case, we don't want to steal page focus from Google, so we introduced a new state: the Urlbar view opens without the Urlbar being focused. The view still only contains the one tip result, encouraging the user to search with the Urlbar instead of with the search engine homepage. Mouse users can either ignore the tip and continue searching with google.com, or they can click the "Okay, Got It" button at which point the focus changes from the Google search box to the Urlbar.
As it stands currently, upon seeing a Search Tip on a search engine homepage, to accept it a keyboard user would have to focus the Urlbar (Accel+L), down arrow to the tip result, and press Enter on the "Okay, Got It" button. The only result of this action would be that the Urlbar is focused (again).
Search Tips are indeed meant to interrupt sighted users to some extent. Certainly more than CFR. And yes, they aren't modal.