Recently, the Firefox browser always prompts that the Flash plug-in needs to be updated, which affects your experience of browsing the website.
Type "about:config" in the address bar of Firefox -> click the "I promise to be careful" button
Enter "extensions.blocklist.enabled" and switch to false. 
Find "Options" -> "Advanced" -> "Updates" -> select the option "Do not check for updates"
In this way, the Firefox browser will no longer be prompted to update and can be used normally.