I switched to Chrome, deleted the library, re-imported it, re-included it, and I'm still getting the "No such file or directory" error at compile.
Worried that it might be something to do with my sketch, I tried creating a new sketch and including the library; same error persists.
Any other ideas? As mentioned, I know I can work around this by installing the desktop IDE, but it would be nice to be able to get this working on the Online Editor.
Thanks again for your help!