Imports from and Exports to recent versions of Firefox 4 beta will fail with Error 453008 if you are using the current "production" version of BookMacster, version 1.1.10.  To fix this problem, update to the latest beta version of BookMacster.

To update to the latest beta, click in the menu BookMacster ▸ Preferences ▸ Updates, check the "Beta Versions (Early Adopter)" box, then in the menu click BookMacster ▸ Check for Update and click through the update process.

We'd appreciate you reporting any bugs you find in BookMacster beta promptly, either here or via email whichever you prefer, since we'll need to publish this for all users pretty soon.