Despite all its shortcomings, there are still times when you want Adobe's flashplayer. Here is one way to install it locally:
Create (if necessary) the directory "~/.mozilla/plugins".
Go to https://get.adobe.com/flashplayer/
and select ".tar.gz for other Linux" for download.
In the top level of the downloaded archive, you will find the file "libflashplayer.so". Copy that file to the "plugins" directory.