Kind of off-topic, but is a two year old Mackbook Air having problems with just browsing the web? Because if it's not a "top-of-the-line computer", it sure costs more than one.
This thread spurred me to do some testing, and it turned out that the "HTTPS everywhere" extension was slowing down pageloads in Firefox by MULTIPLE SECONDS. https://www.tithess.gr/ loaded in 1.3 seconds in Chrome and 6.8 in Firefox. Disabling HTTPS everywhere brought them to parity, but I'm going to switch to Vivaldi, I think.
I've been having problems with slowness for years with this. I dreaded having to open the browser every time. Hopefully now it will be a better, and it's looking much better so far...
