With Firefox, I got to over 600 tabs on my 2011 Dell desktop (Core i5-2500) with 4GB of RAM. Because of "lazy loading", I'd guess that only about 40 or 50 were actually loaded.

RAM use varies from about 1GB to 2.5GB. It slows down above that, which you can only solve by restarting the browser.

I bought another 8GB so I reckon my 2011 machine will be good for another few years. It should last a decade, really.