Good idea brother Richard. Too often I just assume people know things when then they don't.
Firefox is considered what people call "open source" software. Meaning that the source code of the software itself is available to look at. This allows people with programming knowledge to look at the code and see if there is anything "bad". This cuts down on spyware and malware. It also allows you to make changes to the software if you wish. And is 99% of the time free to use.
In some "open-source projects" like linux lots of different people give their input and in a way create software that comes from a group of people instead of an individual.
The idea is to create free quality software. Because sometimes people just can't afford the commercial products.
You can read a bigger and probably better definition of "open-source" here at Wikipedia