I've discovered another problem with WP 3.5 compatibility. The way this plugin is supposed to work is that if you hit the Enter key once, a BR tag is inserted. If you hit it twice, a P tag is inserted. That's how it worked in WP 3.4, but not in 3.5. Instead, in Firefox, hitting the Enter key once puts in two BR tags. In IE, only one BR tag is inserted, as it should be, but hitting the enter key again adds a second BR tag instead of putting in the P tag it's supposed to. I don't know about other browsers.
Just in case you're wondering, yes, I had the New Line Configuration set to "Both."
Having two BR tags instead of a P tag isn't acceptable, so I had no choice but to change the configuration to P-Tag, although this completely defeats the whole purpose behind me installing this plugin, which was to enable the insertion of single linebreaks without going into HTML mode. If this bug can't be fixed, I might as well just disable the plugin.