Why a collection of multiple versions of Firefox is useful to a web developer is pretty obvious, so let me explain why I think the package is "too much".

Unlike with IE, Firefox users tend to update their browser version more often. The integrate update system helps in this regard.

This results in the majority of a website's Firefox visitors using recent versions only, such as FF 29 and above. This means a developer could simply skip testing his website on earlier versions of Firefox and focus on the majority, example here being from Firefox 29 onward.

My suggestion to the developer of UtiluMFC is to also build a lighter version of the pack, which contains the x latest versions of Firefox only, which results in a smaller sized pack, easier to download. Otherwise, with newer FF versions added constantly, the pack size will just grow exponentially.