I really like 1Password as a service, but this addon is severly lacking. The overall functionality is there, but several small hiccups make it rather cumbersome to use.
1) It displays a little icon in form fields that you can press to access its functionality. When 1Password is locked (after inactivity) clicking this icon does not in fact open 1password, but displays the message "use ctrl+period to open password". Why would I click the icon if opening 1password wasn't already what I wanted?! Maybe this is a security feature, I don't know? But then make the icon reflect whether 1pw is currently locked or not, so I don't waste time clicking this icon when most of the time doing so achieves absolutely nothing.
2) The 1pw dropdown interferes with firefox' native dropdown in annoying ways. After about one year of usage I still haven't figured out what makes the 1pw dropdown appear and what not.
3) Sometimes when the 1pw dropdown opens up, it makes the whole page jump/move vertically. What's that about?
There are other glitches like this depending on how sites are programmed. Sometimes it's 1pw filling in the fields that makes the whole page jump too.
4) As others have said: It would make a lot more sense if a new password could be saved AFTER the sign up process on a site.
5) There is no way to customize the password generator as there is for exampe in the Android app. If the suggested password doesn't meet the site's criteria you're out of luck using the generator.