Rated 5 out of 5 stars

Can this be used with 1Password (working fine on SL/alpha3)? I assume this would be a bad thing?

This review is for a previous version of the add-on (1.0b1). 

Without knowing how 1Password does its integration, I couldn't say for sure. I imagine it must intercept Firefox's normal password mechanism so it probably works fine. But if it's overriding Firefox's password mechanism, there's no point having an extension that changes the backend used by that mechanism. My guess is you would see no change in behaviour with this extension loaded if you are already using 1Password.