ContentSearch.jsm is already nice enough to broadcast information to ContentSearchChild any time there are changes to the state (the state being the list of search engines and which is the default one).

However, ContentSearchChild doesn't do any effort to keep this information cached locally, and requests this information again every time a new about:newtab tab is open.

Given that this cached information is kept in the privileged content process, and that process is kept alive [1], this is even more incentive to keep the information cached in the child. It would avoid an expensive msg round-trip for every new tab open.

[1] https://searchfox.org/mozilla-central/rev/dc0adc07db3df9431a0876156f50c65d580010cb/modules/libpref/init/all.js#3379

If I understand this correctly, then I think this is probably either a matter of caching the CurrentState and CurrentEngine messages in ContentSearchChild.jsm.

Unless there's a way to cache the ContentSearchUIController._engines and ContentSearchUIController._defaultEngine values in contentSearchUI.js.

I think once bug 1704156 is fixed, this will no longer be necessary.

Closing as a WONTFIX because it is highly likely that we'll remove the in-content search box code.