How do you install addons from a site other than mozilla addons ?
A normal person will click on the link to extension's
.xpi file. Firefox will then nag with a message bar on top saying it prevented the site from installing software in your computer. Then one have to click the Allow button to install the addon.
In the background, this process adds the site permanently to your list of sites that are safe to install extensions in your computer, which may not be what you intended, if you were just trying out new extensions. Doesn't look too good. Isn't it?
Well, it's time to stop being just another normal person and become a real uber-elite Firefox user. So read along the different ways in which one can install an addon in Firefox.
All these methods do just the same thing - install a Firefox extension. You have to the select the method that best suits you and the current situation and adopt it. Eg: If you have downloaded a large number of extensions, enabling the Install button may be worth it.
File -> Open File and paste the address into the location field. Hit enter.
about:config into the address bar and hit enter. Click "I'll be careful, I promise!" button. Change the value of
extensions.hideInstallButton to false. Now, if you take the
Tools -> Addons, you can see a new button named "Install" near to "Find Updates". On clicking it, a file-open dialog is shown. You can then browse to a downloaded xpi file to start installation.
The usual disclaimer is valid here - "Never install an extension from an untrusted source".