Firefox add-on allows you to virtually run any Google Chrome and Opera extension inside Firefox.
"Enables the Google Chrome Store and Opera Addons Website for Firefox. Point and click to install Opera/Chrome extensions straight into Firefox."
100% free.
My comment: Once I've discovered Chrome Store Foxified I have immediately installed it, and have not removed ever since. A very useful, indispensable utility for Firefox. While it is not perfect and it will not run many Chrome extensions, it still remains an extremely useful tool.
Try it out now: https://addons.mozilla.org/it/firefox/addon/chrome-store-foxified/