Adblock Plus blocked from loading

I've tried all kinds of suggestions from Mozilla to be able to get Adblock Plus to download, but no joy. The error message says that it is unsigned, therefore it can't be downloaded. I've attached a screenshot of the error. It's such an automatically appearing message that it has to be some kind of a setting, but I can't figure it out.
Thanks.

 

Hi plodr,
You're right. It turned out when I updated FF, I was able to get the addons despite the warnings.

Do you know what constitutes a "signed" or "unsigned" download? Is this a registration process with some entity somewhere or how does software get a signature?

 

The problem I ran into was that it was a site I trust, but I couldn't download the file because some setting in my computer was blocking the download. After updating my computer with both SP3 and a more recent version of Firefox, I was able to download it. I thought maybe by learning more about what signed or unsigned is, I could figure out why or how the computer might have been blocking me from having the choice to download something.