I haven't carefully profiled memory use, but in my experience, Chromium is so much more performant than Firefox on ARM devices that any difference isn't worth it. If you're using a lot of tabs, it might lean in Firefox's favor, but overall performance so strongly favors Chromium that I've given up trying to use Firefox on anything but my high performance machines. I'm not sure where the performance delta is coming from, but the whole UI and JavaScript anything are much more responsive on e.g. A73 cores with 4GB RAM.