It's been a while since this topic was being discussed, but replaying LS2 reminded me that we still don't know all the sylvari firstborns. At launch, we knew 9 out of 12, later Wynne's fate got revealed, but two are still completely unknown.
In LS2, Caithe's memories are especially interesting as they give us a bit of a timeline. Apparently, the two missing firstborns disappeared even before the secondborn awakened. Riannoc was already dead by the time of Caithe's first memory, so it's not certain if they disappeared before that happened or after. Also, this happened before the Nightmare Court was founded. At this time, the death of a single sylvari was a huge deal for them all. So I imagine if at a time like this two more firstborn would have died / had something traumatic happen to them, it's likely we would have learnt about this, since it should have left a strong impression on sylvari society, especially the other firstborns.
In contrast, when Wynne did die, the sylvari had already experienced trauma and deaths as a society numerous times. This was some time after the asura captured larger groups.
So, because they disappeared at such an interesting time in sylvari history, I personally suspect they might be still alive and on some kind of mission. Maybe exploration or diplomatics concerning other continents, so they basically had no chance to visit home in the meantime? Like, I don't know, maybe Cantha or even that continent in the west that seems to have trading routes connecting it to Central Tyria? However, sending them that far away when their siblings even barely left the grove... I'm not convinced.
... Or maybe they died in some really stupid accident that everyone refuses to ever mention.
Has anyone heard anything about them in the past ~8 years? I mean, they must have some kind of story, otherwise the devs could just have not defined the number of firstborns in the first place.