IndexedDB - ObjectStores vs multiple databases vs indices?
As long as there is no cross transactional manipulation among object stores, you can separate them into multiple databases. I prefer to use separate database, as many as possible, so that schema changes are easier in smaller object store database.
In rare situation, I even use separate database even if cross transaction is require. These cases are found between user setting database and application database. Inconsistency between user setting and application are fine, since the truth is in user setting and temporary inconsistency does not matter.
Notice that there is high cost associate with opening a database. But once it is open, there is no memory consume for the connection. There is no limit in number of databases.
Multiple databases will have higher thourghput than single database with multiple object stores, since implementation of Firefox lock down whole database on any write transaction.