Should this just be inside the companion object of RecentSyncedTabFeature? No strong opinions on this, but it might be something worth doing to make it testable.

I hadn't thought of it from a testability standpoint. A companion object would probably make the most sense in that case. I generally kinda dislike the pattern of instantiating a whole object just for a const val, this SO post covers why a bit. A top-level internal val could work if we had modules in Fenix, but I think namespacing it with a companion makes more sense since we don't.

Hmm.. interesting. I don't know if we really considered the pattern about how we define constants. My initial reaction is simply based on what I have seen in the project, and looking at what we did in the BookmarksUseCase. I won't be the best person to weigh on how we should ideally proceed with this, but might always be something to bring up in a discussion if you think it's worthwhile to establish some sort of guidelines.

I am not sure if 3 days might be too short. We have used 10 days in https://searchfox.org/mozilla-mobile/source/fenix/app/src/main/java/org/mozilla/fenix/components/bookmarks/BookmarksUseCase.kt#107. It's okay if you believe this is enough, but I figure I would ask anyways.

I used 3 here to try and keep it a little more performant, we're already struggling with the performance of this feature a little bit. My reasoning being that typically a "recent synced tab" has probably been visited on its original device within the last 3 days

Thanks, sounds good to me.