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I'm having a lot of left over plugin container threads after closing. Why?

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When I close Firefox, I have leftover processes. I have 2 plugins, but I usually have 5 - 15 plugin-container processes that do not close when Firefox closes. There also is an instance of Adobe Acrobat that Firefox starts on closing. Why is this happening??

When I close Firefox, I have leftover processes. I have 2 plugins, but I usually have 5 - 15 plugin-container processes that do not close when Firefox closes. There also is an instance of Adobe Acrobat that Firefox starts on closing. Why is this happening??

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That can be caused by clearing the cookies via "Clear Recent History".

  • Firefox/Tools > Options > Privacy > Firefox will: "Use custom settings for history" > [X] Clear history when Firefox closes > Settings

If you use "Clear Recent History" to clear the cookies then Firefox also tries to clear the cookies from plugins like Flash (LSO) and starts a plugin-container process for each plugin and that can add extra time and can cause an unresponsive script error to appear.
Make sure that all plugins are up to date.

Exclude the cookies from "Clear Recent History" to see if that helps.
Let all cookies expire when you close Firefox to remove unwanted cookies and keep the ones with an allow exception.
Make sure that you do not clear the "Site Preferences" as that will remove all exceptions.

  • Tools > Options > Privacy > Cookies: "Keep until": "I close Firefox"