Manually Installing Extensions in Firefox from Mozilla Update

On the Extension's download page, right click on the Install Now link.
A download dialog box will appear. Click Save As... and save the Extension's .xpi file to your local disk (usually your Desktop).
Once the download completes, find the Extension's .xpi file, from the File menu, select Open File..., and open the saved Extension's .xpi file.
A Software Installation dialog will appear. Select Install Now to start the installation.
The Extension Manager will appear, and the extension will be displayed. Restart Firefox to complete the installation. The Firefox Extension has now been installed.

Installing Extensions