Windows 11 is enshitified enough to be annoying and loaded with government and corporate spyware enough to be alarming. Meanwhile, Linux has become user friendly, optimized for everything but the latest hardware and capable of running most games.

I don’t know how the timing of each release is planned but the Firefox website gives instructions for using the repositories for esr, beta, nightly, or dev edition.

Using Debian as your distro doesn’t lock you into firefox ESR.

I don’t know anything about Mac hardware, but if it’s possible to put another 2gb of RAM in there and an SSD, it should be fine for web browsing.

I did something similar for a friend. I spent about $25 on hardware to put equivalent upgrades in an old laptop that had a single core celeron processor. I installed linux mint xfce. It ran firefox fine but couldn’t quite play smooth video on YouTube. It was otherwise usable. A dual core should be workable.