ianmcqui, to BC forums!
I installed the same Firefox extension on my Linux Mint 17.1 OS, and all of the "http" sites shoes as insecure, and the "https" ones showing varying levels of security. This is probably an extension to help those ensure transaction sites are secure, and to what degree. For example, eBay had an 18% score, while Amazon had an 88% one. The eBay results were a shocker to me, as I purchase a lot there.
As to browsing security, it's been known for years that https sites are more secure than http ones, however that doesn't mean that all http sites are unsafe. Being that you're running a Linux OS, you're much more secure than the average computer user. Security is a two way streak, and Linux has you covered. Windows users has to install security.
However one thing that you can do to greatly enhance your own security, is by opening your Terminal, and type "sudo ufw enable" (w/out the quotes) & then enter your password & press Enter. It'll then show that your Firewall is enabled and will run at startup.
The other thing is, all sites aren't created equal. Some sites are dangerous to visit, while others (like this one) is safe. You should install the Web of Trust (WOT) extension to Firefox, this will show you (in general) which sites has good (green), medium (yellow) & bad (red) reputations. I use this tool on both Firefox & Chrome.
The tool that you're using is the wrong one to go by as far as general Web browsing, anything other than "https" sites will show as insecure. I'm not saying it's a bad tool, just not the one for these activities.