This might just be me, but it seems that this is not a prompt from OGS but some kind of firefox login saver. I believe you will find the problem lies on the pw saver rather than with OGS…
…also, I’m not sure I’d recommend using a browser’s built in login credentials saver.
If you like the convenience of having your passwords saved, I highly recommend LastPass (which I personally use) or any other popular encrypted password manager. They are much more secure and have many useful features for helping you remain safe online.