Is it possible to modify/edit, already installed firefox addons? Where are the addons stored in the computer... I mean which folder etc...
The addons/extensions are stored under Linux in: $HOME/.mozilla/firefox/profile.default/extensions
. The profile
part of the path will be different.
It is possible to modify them if you find the path where it is installed. You would have to restart Firefox everytime you make a change to these source files though. The uninstall function cleanly deletes the addon from your extensions directory and you can view and delete preferences you have set for the extension in the 'about:config' page.
On Windows, that path is usually c:/Document and Settings/(your username)/Application Data/Mozilla/Firefox/Profiles/(usually default)/extensions
If you delete a folder(extension) from the extensions directory, it has the same effect as uninstalling it.
You will want to find your profile folder (instructions) and then find the folder called "extensions". In there, there will be a folder for each add-on you have installed.
Note that each time an add-on is updated, your changes will be overwritten.