Fullsize Image viewer for GooglePlus 5.1-signed No Restart
Install it --> Right click on any image when you on Google Plus --> Click on "Full-size" item.
This will open a full-size image in a new tab. Works with any image in your stream (but not for the external links), thumbs and small profile images.
This add-on has been preliminarily reviewed by Mozilla. Learn more
This add-on is not compatible with your version of Firefox because of the following: