[dillera]

I'm not sure about that. I got my wife the 1st gen MacBook, and I have a 2012 i5 11" MBA. I like the size of the Macbook, I don't mind the keyboard but it's very much laggier than the 2012 MBA. MUCH. Once you have something running it's ok, but sometimes starting an app like FF the MB will sit there for 5 sec with no feedback. I haven't seen Mac's do that since the PPC days.

[Razengan]

Well, Firefox currently seems to have the worst performance of all the major browsers on macOS. Try running it in a fresh environment, after backing up and removing ~/Library/Application Support/Firefox

However, while Safari always runs a lot better even with a lot more tabs, Firefox on my 1st-gen MacBook definitely doesn't take a whole 5 seconds to start. Could there be some resident third-party crap running in the background on yours?