Is it possible to “fsync” downloads with Firefox (really Librewolf, but still) so that it goes directly into storage instead of RAM?

Fsync will write data that your userspace program has already handed off to the OS. This probably won’t be your problem; your OS’s kernel should already write out that data if it’s under memory pressure. Assuming that this is Linux, you can twiddle Linux’s kernel settings to make it write out more frequently. The sync command will also write out existing data that hasn’t been written to disk by the kernel, and you could do something like $ while true; do sleep 1; sync; done.

But you probably have an issue inside the application, not in handling data that needs to be fsynced.

I’d also add that I’d be a little surprised if this happens with Firefox’s native downloading. Maybe someone could have some sort of custom download Javascript something-or-other that stages stuff in memory on a website, though I don’t think that even a poorly-written Javascript should be able to produce a tab crashing.