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I thought that was an interesting choice for Mozilla, too. Here is what appears to be the bug report which contains their debate on whether or not to use codemirror.

https://bugzilla.mozilla.org/show_bug.cgi?id=816756

I would be curious to know if Mozilla is still actively involved with Ace development at this point?

Codemirror is very polished and has great support. (It's also used in the Chrome debugger.)

CodeMirror is significantly easier to work with than Ace.

The author of CodeMirror (Marijn Haverbeke) is also a Mozilla contributor to some degree (and, early on, was employed by Mozilla Research on the Rust project).