Piclens is like a photo viewer that hovers in the background and activates when you are browsing pictures on Firefox. It allows to view them as album with a slideshow and full screen. Slightly large in size (1 MB) but a fantastic utility nevertheless.
Installed another recommended one - Tab Effect . It adds effects when a current tab is changed. But it got a bit boring after a while. Have disabled it now.
The baap of all is FEBE backup add-on. The brilliant thing about it can back up all the add-ons, themes, preferences, bookmarks, history, permissions. I have configured it to backup just my add-ons, so I can install them on other desktops that I use. The restore can be done with one click of the button.
Here is the status report at the end of the back-up.