I remember back then when people stop using FF because it used more PC resources than the OS itself and all started using Chrome because it was fast and lightweight.
Mental how it is genuinely the other way around now, but on the masses people might not even know that a computer has limited resources so that's probably a contributor to no mass exodus to FF.
The average person definitely doesn’t have a good understanding of computational resources, but they will use an application they find smoother and less clunky than another. Realistically the performance and resource usage of chrome is not going to be bad enough to drive most people to Firefox these days, and Firefox won’t be enough of an improvement for most people to notice. Chrome also had a huge marketing campaign when it launched… I suspect that was crucial for getting people to adopt chrome (otherwise how do you even get people to think about switching?), but I don’t think Mozilla has the resources for such a campaign. Time will tell, though. I hope we’ll see more people switching to Firefox in the future.
Not necessarily. Using more RAM doesn't increase energy usage, at least not significantly. And if you can use that to avoid making disk or network accesses, it'll save energy. Obviously keeping the CPU spinning at 100% isn't helping anybody, though.
If it forces you to buy more RAM, it does. I think most notebook laptops have had their RAM specified based on browser needs those past years as it became the main application by far.