Is there a way to install a deprecated addon in Firefox?
I remember doing it in Firefox 3 - can't remember how I did it exactly :(
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I hope you are asking about incompatible add ons. If that's so. Install compatibility reporter firefox add on and install your Firefox add on. Now it will be installed!!!
While the currently accepted answer is still accurate, it has become a lot more cumbersome with the ever increasing number of newer versions of Firefox. Granted, Firefox handles compatibility better than it used to, and the need to force enable deprecated add-ons is less common. However, in case someone still needs to do this and finds this question, I'd like to give a more modern, easy-to-use answer.
Install the add-on Disable Add-on Compatibility Checks. Then install your deprecated add-on. Before restarting Firefox, be sure that no add-ons you do not wish to use are turned on by checking the Addons panel. After restarting, see if the add-on works. If not, disable it again.
It is possible that Firefox will break in a way that you cannot get to the Addons panel and shut off the add-on. If so, close Firefox by using Alt-F4, wait for it to close completely, and then start it again in Safe mode by holding down shift while you click to open Firefox. Disable the addon in the Addons panel and then restart Firefox.