What is an installer? An installer is an executable program that will facilitate the copying of new files into your hard drive. These files are usually in a compressed form to lessen the total file size of the installer, in order to facilitate a faster rate of distribution when it comes to spreading the copies of the installer. These files are the components that the installed software utilizes in order to run its program.
If you go to the Mozilla Firefox website and click the download button, you will be downloading a file named Firefox Installer. When you run this installer you will notice that it will not immediately have Firefox installed, because it still has to download the files that are to be used during the actual installation process. Therefore what you actually downloaded was more of a download manager than an actual offline installer.
If I can install Firefox Mozilla Browser straight from the Internet, why would I need an offline Installer?
The answer is simple. Imagine a scenario wherein you will have to prepare and setup a network of computers. And one aspect of the whole setup is that you will need to install applications that are going to be used with Mozilla Firefox being one of them. Sounds simple enough right? But what if you will need to do this without Internet connection or there is one but the speed is not optimal. It will take more time for you to install Mozilla Firefox on each computer if you still have to download from the Internet for every computer unit. That is not a good use of time and resources at all and so an offline installer must be available for download for key applications such as Mozilla Firefox.
Where can I download the offline installer?
Navigate to this webpage where they list all the possible languages for the offline installer:
If you want to know more about what happened between the adobe flash player and the different browsers, check this out.