Dave, yellowed plastic is usually not caused by fire retardants.
I think the bromine myth may have been started by Nintendo. Apparently they had some controllers that yellowed quickly and had to come up with an excuse...
If you want to test the "bromine" hypothesis, maybe go outside and set fire to some pieces from old equipment and show us the results. FR plastic should self-extinguish in a few seconds - ideally as soon as you remove the flame. If it keeps burning, then it's just the regular stuff.
Proper fire retardant plastic is harder to mould, plus it's extremely expensive (like add a zero expensive!), so normally it's only used where regulations demand it.
For consumer products that are "built down to a price", manufacturers want the cheapest option possible. (I suspect this is why many early PCs were that consistent beige/white colour - that's the natural colour of ABS. Adding colourants would have made them more expensive to produce.)
ABS is particularly prone to yellowing. The polymer molecules are not completely stable and prone to oxidation. Their structure can be modified by absorbing a high energy photo, exposure to heat, or background radiation. (Our DNA suffers the same way.)
The resulting molecular changes cause the plastic to preferentially absorb more blue & UV light (which accelerates the process), leaving just the green & red to get reflected - which is why it looks increasingly yellow.
It happens to clear polyurethane as well. You may have noticed that the tubing used on industrial air lines also goes yellowy-brown over time. And the same thing occurs to the cellulose polymers in old paper, which is definitely not fire retardant.