Your information is outdated. It hasn't been called IceWeasel for a while now and that only had to do with a trademark issue[1] but it was still based on the Firefox sources. Source: running Firefox via .deb (firefox-esr in the Debian repo) on Debian stable to write this.

[1] IIRC, it had to do with using the Firefox name even though Debian was lightly modifying sources to remove a couple of things. This was resolved a while ago and the Firefox branding resumed.