You'd need to verify it in Thunderbird 71 beta 1 from here: https://www.thunderbird.net/en-US/thunderbird/beta/all/ Open a compose/Write window and press Ctrl+F. Find bar appears. Trust, me it works ;-) I don't know of a way to verify it in Firefox.
Bug 1589519 Comment 13 Edit History
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You'd need to verify it in Thunderbird 71 beta 1 from here: https://www.thunderbird.net/en-US/thunderbird/beta/all/ Open a compose/Write window and press Ctrl+F. Find bar appears. Trust me, it works ;-) I don't know of a way to verify it in Firefox.