Look at the description for the field you are filtering on. Doesn't look right, but does give you an idea of where to look:
ID of the location where the version was first published. If the version is first published into a user's personal library or My Files, the field will contain the ID of the user who owns the personal library or My Files. If the first version is published into a public library, the field will contain the ID of that library.
Accepts all record IDs supported by ContentDocumentLink (anything a file can be attached to, like records and groups).
Setting FirstPublishLocationId allows you to create a file and share it with an initial record/group in a single transaction, and have the option to create more links to share the file with other records or groups later. When a file is created, it’s automatically linked to the record, and PublishStatus will change to Public from Pending/Personal.
This field is only set the first time a version is published via the API. FirstPublishLocationId can’t be set to another ID when a new content version is inserted.
ContentDocumentLink is what you are looking for instead.
WHERE ContentDocumentId IN (
SELECT ContentDocumentId FROM ContentDocumentLink WHERE LinkedEntityId = :accountId
If joins don't work (which wouldn't surprise me), you may need to add it as a separate query instead.