My 10 year old bushes are turning yellow and dying.
Certified GKH Gardening Expert
This could either mean that it needs to be fertilized with a high nitrogen fertilizer, or that the pH of the soil is off. First try putting down some dolomitic lime, and if this does not correct the issue, then fertilize the shrub.
Here is an article for more information: https://www.gardeningknowhow.com/ornamental/shrubs/firethorn/care-of-firethorn-bush.htm