When installed this add-on will colorize your tabs depending on the protocol they use (currently [http/https red/green respectively] only). So you would know if you are browsing through a secured connection even if you have hidden the navigation bar.
Here is more information about this add-on:
If you are (like I am) one of those people concerned about making better use of screen space, or you are otherwise limited by your netbook/laptop screen size, then you know how important is to take occupied space to its minimum while retaining as much functionality as it is still possible. For this reason I have created this simple yet useful addon that will keep you visually aware of the transfer protocol being used by each of your currently open tabs. Currently there is no possible configuration to adapt its visuals to your individual preferences, if it gets audience I will quite like to keep improving it though.
This is a great idea and a great add-on. Pick up the Security Tab Colorizer from the Firefox Add-ons website.