In the Forgotten Realms, the Sea of Fallen Stars is a huge inland sea. I'm just wondering if it's saltwater or freshwater?
Seawater is colloquially used to mean saltwater. While there are a few named seas that aren't saltwater and oceanographers do not use sea scientifically - most seas are saltwater. What makes a body of water salt versus fresh water comes down to 3 factors: how big is the body, how old is the body, and how many tributaries enter the body.
The Sea of Fallen Stars has many tributaries, is old, and is very big.
Additionally, while it isn't explicitly stated, much of the material for life in the Sea of Fallen Stars designates if it is fresh or salt water life.
(Originally from Elminster's Ecologies)
Two varieties of this kind of troll threaten the coast: freshwater and saltwater. The freshwater trolls are called scrags (also river trolls), the saltwater are called marine scrags (and sometimes sea trolls)...
Typically when a setting is written, anything out of the ordinary is stated. Often things that aren't stated can reasonably be assumed to conform with the most common versions of those things. We don't describe what legs are when we say a creature is a biped, for example.
So given these 7 pieces of evidence: that people often mean saltwater when they say sea, that none of the material seems to state that it is freshwater, the assumption that most seas are saltwater, that the sea is big, has many tributaries and is very old, and that life is described in the area as being both salt and fresh water - I'd say that it is very likely that the Sea of Fallen Stars is intended to be saltwater.
According to the Advanced Dungeons and Dragons, 2nd Edition supplement Sea of Fallen Stars (1999), the Sea is mostly saltwater with some freshwater:
Most of the races within the Inner Sea breathe only saltwater, since that is more than 99% of its total volume. However, near the coastal shallows and at inlets where rivers drain into Serôs, brackish mixtures of fresh and saltwater occur. (31)
The Sea of Fallen Stars does not have any rivers leaving it, only rivers entering it. This means it only loses water through evaporation. This process leaves behind any salt so it should build up over time.
The Caspian Sea is a real life example of such a situation and it is quite salty, though not as much as the oceans.