Does this fit FSA 90?

I can confirm this is compatible with your model. Along with the below: 4002 710 2108 - Stihl Mowing Head AutoCut 25-2 (https://www.lsengineers.co.uk/autocut-25-2-stihl-strimmer-head.html) 4002 710 2167 - Mowing head DuroCut 20-2 (https://www.lsengineers.co.uk/mowing-head-durocut-20-2-genuine-stihl-oem-no-4002-710-2182.html)