Manjaro 15.12 Deepin comes with Chromium browser installed. Its a good web browser but I prefer Firefox than Chromium. So, I switch to Firefox now. Installing Mozilla Firefox on Manjaro 15.12 is pretty easy. I am using the new Manjaro 15.12 Deepin edition.
If this is a freshly installed Manjaro, we need to refresh the database first. Open Add/Remove Software and press reload or refresh button. If required, you can also set the preferences on Add/Remove Software, change the mirror location, enable AUR repository etc.
Once the database is refreshed, now type firefox to search the Firefox package.
Right click firefox and select Install. Press Apply to confirm the selection.
Press OK and the installation will take place. When I wrote this tutorial, Firefox hits version 43.0. It has many improvements including bug fixes and speed improvements.