De facto, KM and FF are simply different shells, wrappers for Gecko engine.
All FF GUI is XUL-based (->cross-platform but heavy) and KM uses Windows system libs (mono-platform but light).
Moreover, KM has its own macrolanguage, making possible to create some added functionality not eating much of system resources.
FF needs to integrate new XUL-extension where KM can sometime use text
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