It just mean for that page you loaded, the content or whoever wrote the page is calling some code that can make whatever information you fill in visible for others if they wanted to hack or do something with it. Like if you were using public wifi and there is a smart computer guy there can see what info you're putting in that page.
Make sure your firefox is latest version.
If you give us the site name and we visit it we will likely get the same warning. It should not be a virus or anything on your computer at all, it has to do with the site. Usually code that affects encryption are javascript, java, flash, ssl and related kind that can trigger security stuff.
