You got some things right. The issue was never over the main body of source code that Firefox is built from. It is over the artwork distributed with it, the terms of use of the trademark "Firefox", some small bits of non-essential proprietary code like the feedback tool, and the distribution and recommendation of proprietary plugins. Some of this is explained on the
Gnuzilla/IceCat homepage. However, that page is about a GNU project, not the original Debian Iceweasel, for which I can't seem to find a homepage.
This is probably the closest. I'm not entirely clear on some of these details.
Also, it quite incorrect that Debian requires everything to be GPL or LPGL. In the
Debian Free Software Guidelines, it is made clear that GPL, BSD and Artistic licenses are examples of acceptable Free licenses. The DFSG seems quite similar to the Open Source and Free Software Definitions. All three of those definitions/guidelines accept the MPL AFAIK.
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