Jetfire left Cybertron back when Megatron was still leading the Decepticons and that was thousands of years ago, but the alt-mode he scanned on Earth didn't exist until around 40 years ago so he couldn't have arrived on Earth before then. What was he doing during that gap?

Read his Tales of the Fallen issue.

Basically, he went around finding stars to harvest. However, the Fallen somehow already found Earth and sent Jetfire and some Constructicons there, despite the presence of humans.

Jetfire tried to stop the destruction of the Sun, but spent his last bit of energy taking out (a) Mixmaster but teleporting to the upper atmosphere and body slamming him in the (Mojave?) desert. Later, he wakes up in the 60s or 70s while a SR-71 is being test piloted. It crashes, but he scans it and transforms, to the ejected pilot's surprise. We can assume he never transformed again, and went totally into stasis nap once he was put in the museum/decommissioned.

Basically, he missed all the Megatron stuff; he was only a real "Decepticon" for a mission or two under the Fallen.

Well that explains it. I couldn't read "Tales of the Fallen" because there's no where to buy comics around here.

What was Jetfire doing before he came to Earth?

Transwarp speed.