Odd password message boxes popping up

[#1]

I periodically have odd messages popping up on my screen when I navigate to certain pages and the pop-ups look like they're connected with my broadband settings. The latest one to pop up whenever I navigate onto a screen that requires my user-name and password is:
"Please enter the master password for the Software Security Device"
I just click on "cancel" and continue as normal.
Why does this box pop up and how do I stop it from doing so?
If anyone has any explanations and solutions, that would be helpful- please avoid jargon and acronyms, as I'm not an IT expert.
Thanks.

[#2]

If you are using Firefox, I would assume so with what you are describing, then it is asking for your master password (set in preferences) so that it can retrieve your previously saved username & password for the site you are accessing.

[#3]

Thanks- I had no idea one was set, as the message has only just started appearing in the last few days, along with a Plusnet one, the contents of which I have forgotten. Problem solved.