As someone who has done a lot of work on performance issues in FirefoxOS, and someone dogfooding FirefoxOS on the lowest-end hardware, I don't agree. I don't know exactly what issues Corbet is complaining about, but most of the slowness/lag issues I see are not GC related.
One big issue is app startup time. This is related to Web platform issues --- it's hard to precompile a Web app --- but not GC.
Another big issue is smoothness of CSS animations and transitions. These are managed by C++ code and GC is not involved.
Another big issue is smoothness of panning with touch gestures. Again these are managed by C++ code that isn't impacted by GC.
(Our JS GC has gotten a lot better since the version of Gecko that's present in these 1.0 devices. We have greatly reduced pause times since then. I don't think that will make much difference to FirefoxOS users though.)