Screenshot.

Extensions that do not have a "Remove" button are installed globally by other software and are not under control of the Firefox extension manager. Globally installed extensions are usually found via a registry scan or are installed in a location that Firefox scans for installed extensions. You will have to contact Kaspersky if you have a problem with their software.

You can find the install location of a specific extension in the extensions.json file in the profile folder. You can open the extensions.json file in a Firefox tab or in the Scratchpad (Web Developer) and search for this extension. The path field list its install location.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

How can I forbid to install extensions globally? KIS support doesn't help. All what they say: "You can disable extension. Why do you want to delete it?"

Neo. said

How can I forbid to install extensions globally? KIS support doesn't help. All what they say: "You can disable extension. Why do you want to delete it?"

Firefox will discover extensions installed in various shared folders and listed in the Windows registry. I don't know whether there is a way to narrow Firefox's gaze to your currently active profile. If you like, you can delete the KIS extension from wherever it was placed by KIS. Do you need tips on locating it? Try the method described in this thread (adapted to the name of the extension you're looking for): Delete legacy extension that has no remove button

Ijscher2000 said

Firefox will discover extensions installed in various shared folders and listed in the Windows registry.

I know that extension is placed here: "C:\Program Files (x86)\Kaspersky Lab\Kaspersky Internet Security 19.0.0\FFExt\light_plugin_firefox\"

How can I forbid firefox discover extensions from folders placed not in my profile?

You should be able to disable these third-party extensions in the Add-ons manager although you can't remove them. You would have to remove the Windows Registry keys that Firefox scans, but it is likely that KIS will restore these key.

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Distribution_options/Add-ons_in_the_enterprise

You can also try to remove the extension(s) from their installation folder.

Thanks for the help. I deleted the extension files in the light_plugin_firefox directory and change the NTFS rights on the directory. I did the same with the registry directory HKLM\SOFTWARE\Mozilla\Firefox\Extensions & HKLM\SOFTWARE\WOW6432Node\Mozilla\Firefox\Extensions.