Answer depends on browser.
Best alternative is to install chromium browser. It includes a built in flash player, no separate client is needed. It replaces the protocol, not just another copy of same browser addon/library.
If you insist on Firefox, there is flashplugin-installer package you can get in multiverse repository. It downloads from Adobe. It puts an 'so' file is a place where FF finds it.
Don't get your hopes up. I've got it in firefox, but it crashes constantly. The problem to remember is Adobe dropped the Linux Firefox add on project years ago. Nobody is trying very hard to make it go. I spent most of weekend testing and trying to fix. It can connect to start session, always fails as soon as you click something (Firefox 55). I can confirm that chromium is good alternative. Interesting, for me, that I don't enjoy chromium, but it is only flash alternative in Linux.