Crossbow Expert states
You Ignore the loading property of Crossbows with which you are proficient.
Could this also apply to firearms, such as the Artificer's Thunder Cannon, or would this be purely up to the DM?
A firearm isn't a crossbow. Feats, like all character features, only do what they say they do. So as written, Crossbow Expert only allows you to ignore the loading feature of crossbows, not any other missile weapons.
Furthermore, of the firearms detailed in the DMG (pp. 267-268) and the Artificer UA playtest material, only two (the Renaissance pistol and musket) have the loading property. Some other weapons have the reload property, which is not the same thing. The Thunder Cannon has neither of these properties; instead it has separate rules about how it is reloaded (using a bonus action).
Your DM may rule that Crossbow Expert also applies, if possible, to some firearms (if they are present in your game), or they might allow a "Firearms Expert" feat, which provides all the benefits of Crossbow Expert, but for firearms. Even then, such a feat wouldn't affect the use of weapons with the reload property, or the Thunder Cannon.
Still, by rules-as-written, Crossbow Expert doesn't affect firearm loading.
The feat is explicit about when it refers to only crossbows and when not. Take the second benefit:
Being within 5 feet of a hostile creature doesn't impose disadvantage on your ranged attack rolls.
This says nothing about crossbows and will work with any ranged attack. So when it does call crossbows out, you have to stick to it.
I would recommend leaving it that way even if custom weapons are introduced, like firearms. But of course if you add a homebrew item, the specifics are up to you.
You ignore the loading quality of crossbows with which you are proficient.
Because it explicitly states that it works for crossbows, it cannot be used for any other weapon other than, well, crossbows.
However, the rest of the benefits provided by taking Crossbow Expert do not state that your weapon must be a crossbow and therefore, implicitly, you would not necessarily need to use a crossbow to benefit from those.