Firefox failed to install a required plug-in on a eLibrary site and I was supposed to install it manually. So when I clicked on manual install I was diverted to a page to download the missing plug-in.After downloading the .xpi file I was not sure how to install this.
After few searches on google I was able to install it. It’s pretty simple to install.. once you have this extenssion just Open Firefox, Select File > Open File > locate the xpi extension > install
Bingo!!! your Firefox extension would be installed
Nilesh's Weblog 
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