They are, usually, not needed for Firefox.
Jim, that was my quote, and I bolded the "usually" part. As far as I can see the .dll files are not needed in Firefox. Firefox seeks the program out of which the .dll files come. I have one installation of Firefox3.0b2 which works, perfectly, well with no other .dll file than the npnul.dll.
In case what I said was not clear, the programs must be installed, but their presence in Firefox is not needed. The Firefox3.0b2 install has all the needed settings for the installed programs (including Quicktime) in Tools|Options|Application. Such settings are also available in FF2.x, in Tools|Options|Content|File types. At least as far as I am concerned.
Also the word "plugin" is, often misused. The installed program is one thing, and a link to it, is another one.
Of course, I may be mistaken, because my default browser is FF126.96.36.199, which has .dll files for a number of programs. From what I see only Quicktime has recent files. All others are 2-4 yrs. old.
By the way, I don't understand the significance of the links given.