Thats why I dont and wouldnt suggest anyone use any experimental addon.
Firefox usually gives a warning to such effect whenever you go to install an addon Legit or not.
Any addon I add i keep an eye on and make sure its been around for a while.
I dont use a tremendous amount of addons anyway.
AdBlock
NoScript (wonderful little addon)
Add Bookmark Here
New Tab Homepage (automatically brings you to your homepage whenever a new tap is opened. In my case Google)
Down them all (enables you do download any or everything on a page at once. comes in handy for the AH skins. Click on the ones I want, Click download, and ZAP! Sure beats downloading them one at a time)
Image zoom -exactly what it infers