Install a Firefox Add-on Manually
Here are a few quick and easy ways to install Firefox add-ons manually. Some are quicker than others, you'll just have to pick the one that you like the most.
Before moving on, I want to recommend about:addons Launcher. It's a simple lightweight add-on that provides one click access to the Add-ons Manager via a toolbar button.
It's the fastest way to get to your add-ons and it has been fully tested and approved by the Firefox Add-ons site.
First Things First
If you have already saved an extension to your desktop (or other location), you can skip this.
Download the extension that you want to your computer. For security reasons, it is highly recommended that you only save/install add-ons from the official Firefox Add-ons site, from an official Mozilla site, from a Mozilla repository on github, or from a trusted Mozilla employee or volunteer.
If a download option isn't available, right click on the extension's install link and select "Save Link As..". Be sure not to change the file extension. A Firefox extension will be an .XPI file.
Once you have downloaded the extension, there are several ways that you can install it.
Tip: To find the location where add-ons are installed by default, press Alt + h > t (Help > Troubleshooting Information) > Profile Folder > Show Folder > extensions.
Manual Install Methods
Since Firefox 4, there has been an option to manually, and easily install saved add-ons.
Open the Add-ons Manger > (Alt + t > a) click on the gear icon (Tools for all add-ons) > Install Add-on From File.. > browse to the add-on > double click the add-on (or select Open) > Install Now
Despite this, the following tips may still be better and/or quicker options for you.
With Firefox open, hit Ctrl+ o (or Cmd+ o) > browse to the extension > double click the extension (or select Open) > Install Now.
Press Alt + f > o > browse to the extension > double click the extension (or select Open) > Install Now.
Drag the extension to the instance of Firefox on the taskbar > wait for Firefox to appear > drop onto the browser window > Install Now.
Drag the extension to the Firefox desktop (or other) program shortcut. If Firefox is running, you'll see the Software Installation window appear > Install Now. If Firefox is not running, it will start up and the Software Installation window appear > Install Now.
If you have a Firefox desktop shortcut with -no-remote -p appended to it (allows for running multiple instances of Mozilla apps), you can install the extension into whatever profile that you'd like if Firefox is not currently running.
If you do run multiple, simultaneous instances of Firefox (e.g., Release, Aurora, Nightly, etc), you can drag and drop an XPI link, or the "Add to Firefox" button (from the Firefox Add-ons site) from one running instance of Firefox to another running instance of Firefox and the add-on will be installed in that version of Firefox (if of course, it's compatible).
Install a Firefox Theme Manually
Firefox 4 and higher.
Open the Add-ons Manger > (Alt + t > a) click on the gear icon (Tools for all add-ons) > Install Add-on From File.. > browse to the theme file > double click the file (or select Open) > Install Now
Open the Add-ons Manger > (Alt + t > a) click on Appearance > drag the theme file that you saved to the instance of Firefox on the taskbar > wait for Firefox to appear > drop onto the browser window > Install Now.
Selecting a Theme
Open the Add-ons Manger > (Alt + t > a) click Appearance > select a theme (single click on one), click Enable, restart Firefox. Your selected theme will be visible after Firefox is restarted.