Firefox plugin gets stuck on "Capturing"

The EndNote Web 2.8 Firefox plugin isn’t working for me. When I try to capture a page, I get prompted for my online account details. When I enter them, it gets no further than this dialog, which I eventually just have to cancel:

Stuck ENW

In IE, the CWYW version in the download section works as it’s supposed to, once I disable the popup blocker.

Any idea what could be going wrong in FF? I’m using FF v3.6 but I had the same problem with v3.5. I’m not particularly interested in capturing to “My EndNote Web”, only the desktop application.