Lucidor, available in Firefox add-on (not compatible with ePubReader; you'll have to choose one or the other) and standalone (less buggy, features work better) cross-platform versions.
Doesn't do that auto-add thing that ePubReader does where it copies a copy of every ePub you open into your Firefox profile folder, though Lucidor does pop up a "do you want to add this to your bookshelf" notice when you drag a random ePub over to it. But that's easily ignored. And I think you might be able to select a preference to make it stop asking that.
Also, you can point it to sets of folders on your computer or a homebrew OPDS catalogue in case you do want a library function out of it.