Define firm. Even when the bushings are shot there's enough tension in the system that the struts feel firm to hand pressure. It might take the weight of the car to reveal looseness. Take a good look at the inboard and outboard bushings of that and the other struts. Is the bolt centered in the eye of the strut?

Another item that goes bad is the bushing between the control arm and the knuckle. I can't find a picture of the fitting but the part is available from Phil.

A quick though inconclusive test is to tighten the lugnuts with the car on the ground (is there another way?). If the wheel shifts relative to the car, that bushing is probably bad. Of course an upper diagonal strut (described above) with worn bushings can have the same effect.

There's also the control arm inboard bushings and the 4 subframe bushings. I'll bet the swaybar end links are a little worn too.

Sixto
87 300D