If you download from mozilla add-on website, after login, click on “Add to Firefox” button and Firefox will automatically ask you to install it. Click on Install button and restart firefox.
If you download from my blog, it is a zip file. Unzip the file on your computer. The extracted file is “pramukhtypepad_ff_25.xpi”. From Firefox, go to File Menu > Open File…. Now locate the extracted file and press “Open”. It will ask you to install it. Click on Install button and restart firefox.
After restarting, it will add Script Menu at the top. You can choose the script you want to type in. This add-on has all the functionality that Pramukh Type Pad has. Pressing F12 will toggle the selected Indian script with English. Also at the bottom right status bar, you will your selected script. This add-on remembers the selected script and when you open up the browser next time, it will automatically select the last selected script for you.
Enjoy typing in your favourite Indian language.