If you want to find out where Firefox downloads and keeps installed extensions in a quick manner, just press together:
KBD Windows flag button + R
This shortcut will open WIndows Run prompt
And paste inside the run prompt
%appdata%\Mozilla\plugins
The %appdata% is Windows internal variable that keeps inside path to C:\Users\Your-Username\AppData\Roaming
On my workPC this contains:
C:\Users\georgi7>echo %appdata%
C:\Users\georgi7\AppData\Roaming
Enjoy 🙂