Where does Firefox put addons?

Where does Firefox put addons?
If I install some of the downloaded addons, where are they placed on a
drive?  

For example, I have an addon called "Download Helper 2.5.3".
It's purpose is to save downloaded videos from sites such as Youtube.
That addon contains a nag screen that begs for money every so often.
I want to see if I can remove the nag screen, but where the heck is
the file?  I looked in all the sub directories in the Firefox dir. and
I can not find it.  I also looked in my registry for ""Download
Helper", and cant find it there either.  I have about 10 addons
installed and can not find any of them.

Thanks for all help.

TJ