Broken ribs are a no no for at least a week.
The issue with broken bones is in the first 24 hours they swell, coupled with being in plaster + a pressurised cabin = recipe for potential dramas.
Hence the requirement for the plaster to be split if you're travelling within 24 hours of the break. (easyJet policy, 48 hours)
However, as you've just stated the break is several weeks old and it's not in plaster, it's ok to travel.
Here's easyJet's
policy. Scroll down a bit.