I'm still seeing this misbehavior in Firefox 114.0.1 (64-bit) snap on Xubuntu 23.04. The workaround for New Tab not supporting file: is the same as the workaround for file: not supporting CORS. Run an HTTP server process in the background on the user's computer serving HTML documents that can be accessed from the New Tab page. The user may want to make sure the server listens only to localhost.
Bug 1607800 Comment 3 Edit History
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I'm still seeing this misbehavior (and similar bug 1580432) in Firefox 114.0.1 (64-bit) snap on Xubuntu 23.04.