Firefox, Chrome and Opera are fantastic browsers in their own right, and are made even better by the wealth of extensions available in their respective stores.
However, for various reasons, not all add-ons are available in the official stores, and must instead be installed manually.
Take care to only install add-ons downloaded from legitimate sources, and remember that you're doing so at your own risk.
You can still install extensions by dragging and dropping a downloaded CRX file onto the Extensions list, but it will be greyed out and you'll be unable to activate it.
Last year, Google extended this policy to the developer version of the browser, closing down that possible avenue.
Drag and drop the CRX for extensions designed for Chrome or OEX extensions developed for Opera file onto the list of installed extensions.