Area under the ROC curve approximation
The thresholds don't matter; what matters are the (FPR, TRP) values at those thresholds, as they are points on the curve. Sort them by FPR ascending. For this to work out, you'll want to include the points (0,0) and (1,1) in your list, corresponding to thresholds 1 and 0.
You can use a trapezoidal approximation, as each successive pair of points defines a trapezoid of area under the curve. You'll just add up the areas.
Let's take two successive points (FPR1, TPR1) and (FPR2, TPR2). The area is (FPR2 - FPR1) * (TPR1 + TPR2) / 2. Just sum that over all successive pairs of points.
Of course, most libraries can compute this for you from this input, like scikit-learn.