If I understand correctly, the point is that icecat is fully foss, whereas firefox is mostly foss, but otherwise they are identical.
I believe that the firefox_getLatest extension will fetch any version of firefox, therefore instead of compiling icecat-60.7.0 (the most recent), you can download a pre-compiled firefox-60.7.0.
..or you can use firefox-ESR (78.1.0) extension compiled on CorePure64, which uses alsa rather than pulseaudio.