[icebraining]

I don't think so, Mozilla browsers never had MDI[1], so they couldn't have copied it from Opera.

[1]: There's a feature request since 2000, but it was never implemented: https://bugzilla.mozilla.org/show_bug.cgi?id=60775

[yareally]

Oh, I wasn't talking about Mozilla (the precursor) in regards to that, just Firefox after ditching the Netscape legacy. Sorry for the ambiguity.