It's not anything you've done wrong. I just reproduced the problem. Looks like a bug. I didn't test the final couple of betas of PasswordMaker 1.7 with SeaMonkey, thinking that an earlier beta test was sufficient (clearly it wasn't). I'll fix this tomorrow and release 1.7.1. For now, please install 1.6.2 or you can manually edit install.js: just unzip the .xpi, change extLocaleNames: ['de-DE','en-US','es-ES','fr-FR','nl-NL','pl-PL','pt-BR','zh-CN'] to extLocaleNames: ['en-US'], re-zip to a .xpi file and drag-and-drop onto SeaMonkey.
Sorry for the inconvenience,
Eric