Thanks for identifying Anitamo; I was hoping this wasn't so, as the bush is so attractive. However, I can see it can be invasive as I have two hardy seedlings sprouting already. I shall keep one for the bats though.

There is more than one orange-berried firethorn on the market, you really need to provide more information to get a useful name here. In fact, most of them would be described as orange-berried rather than red or yellow. If you have a plant or planting near there you are thinking of, maybe you can arrange to get hold of a section of stem with fruit on it (or some good photographs) and take that to local outlets to see what they have that looks the same or similar. The only yellow-berried one I have noticed being in normal outlets here is 'Teton', and upright one with fruits that age to a deeper color, in the manner of Meyer lemons. And I'm not even seeing that one around much anymore. You may have a different mix of cultivars down there, here they have to be scab-resistant for starters.

I only have experience with Ilex verticillata since I decidua doesn't grow anywhere I have lived and isn't common in commerce. Both are deciduous native hollies that prefer average to damp soil. Both have separate male and female plants like all hollies, so yours is female and there is a compatible male nearby since you have fruit. If this is a wild form, you can read about it (or also the various cultivated forms) by plugging in the name Ilex verticillata or Ilex decidua into the MO Botanical Garden's plant finder in the topmost blank on the form. You may find more locally pertinent info at the Ladybird Johnson Wildflower center's online database.

Mulberries? They are white when immature and then gradually turn purple. I'm guessing that because silkworms traditionally eat mulberry leaves, hence "silk berries." (I grew up eating mulberries from the wild trees in our backyard!)