Explain how Firefox uses user profiles

This extends the Firefox profile documentation by explaining how
Selenium works when supplying an existing profile directory. Since
profiles are not cleaned up by Selenium, it also describes how they can
be explicitly removed by the user.

Author: Ozoniuss

Diff for: website_and_docs/content/documentation/webdriver/browsers/firefox.en.md

@@ -135,6 +135,14 @@ options.profile = FirefoxProfile()
 driver = FirefoxDriver(options)
   {{< /tab >}}
 {{< /tabpane >}}
+**Note**:  If you want to use an existing firefox profile, you can pass in the path to that profile. Every time you
+run your program, Selenium will make a new copy of the profile and use it instead of your original one. The copies
+are not cleaned up explicitly by Selenium, they should eventually be cleaned up by the operating system. However,
+if you want to remove the copy explicitly, the path of the copy will be exposed by the `FirefoxProfile` object.
+Check the language specific implementation to see how to retrieve that location.
+
+Please refer to the official [Firefox documentation](https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_how-do-i-find-my-profile)
+for instructions on how to find the directory of your profile.
 
 
 ## Service

Diff for: website_and_docs/content/documentation/webdriver/browsers/firefox.zh-cn.md

@@ -139,6 +139,15 @@ driver = RemoteWebDriver(options)
 {{< /tabpane >}}
 </div>
 
+**Note**:  If you want to use an existing firefox profile, you can pass in the path to that profile. Every time you
+run your program, Selenium will make a new copy of the profile and use it instead of your original one. The copies
+are not cleaned up explicitly by Selenium, they should eventually be cleaned up by the operating system. However,
+if you want to remove the copy explicitly, the path of the copy will be exposed by the `FirefoxProfile` object.
+Check the language specific implementation to see how to retrieve that location.
+
+Please refer to the official [Firefox documentation](https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_how-do-i-find-my-profile)
+for instructions on how to find the directory of your profile.
+
 
 ## Service