Installation of plugins/extensions is quite simple, but the process may seem foreign to someone who doesn’t do it very often and doesn’t know what’s to be expected during the process.
These instructions are applicable if the extension is saved to your hard drive. I prefer to save the plugins directly to my computer just in case the plugin website goes down or to save time if I need to reinstall a plugin (i.e. no hunting down the website where I originally found it).
Open up Firefox and click on “File” -> “Open File” from the menu
A window will open up for you to browse for your file and click on the “Open” button
Another window will pop up and it will appear as if “stuff” is loading, once the button is active click on “Install Now”
Yet another window will open to actually install your plugin, once it’s finished it should say “This item will be installed when you restart Firefox.”
Rinse and repeat 1-4 for any other plugins you’d like to install.
Open up Firefox and your new plugin should be installed.