Tangerine citrus leaf issues.
We cannot say for sure but you may be dealing with some nutrient issues, poor drainage, not enough moisture, sudden changes in temperature, reduced light, etc. We do not see an insect issue. You did not mention how old the plant is, if it was recently brought indoors, and how you care for the plant.
Indoors it needs a minimum of 6 hours of bright light. It is okay to prune off the thorns.
We notice that there is a bag inside the container. The container needs to have drainage holes. Citrus plants like consistent moisture but not waterlogged soil. Choose a lightweight container that has a drainage hole in the bottom to allow excess water to drain out. When indoors, take care not to overwater your citrus plant. Check your plant’s soil moisture once or twice each week. Press a finger down 2” into the soil. If it feels dry, it is time to water.
Fertilizer - Stop fertilizing by early fall. This will prompt your plant to harden off rather than develop new foliage. You do not need to fertilize in the winter.
Take a look at our page on citrus for growing tips and more information
Thank you very much for your very informative answer. The tree is 3 years in our care but was bought in a 3 to 5 gal pot at about half its height. It hasn’t been consistently fertilized and my wife “feels sorry” for it if the soil even looks dry on the surface. When I care for it I don’t water until the finger two inches in test and I water less. They have a bag because they are indoors for the winter but these is a siphon tube to the bottom of the bag, and 4 in of pearlite at the bottom to let the water drain to the siphon.