It goes without saying but I'll be explicit just in case: Browsers will fix discovered security exploits in new versions and document what has been fixed. This means hackers can use the patch notes to see how they can attack old versions of the browser.

One day you will view an ad that redirects you to something that uses an exploit, or something like that. It'll take less than a second and you might not even realize that you've been compromised. That's why it's a good idea to run unpatched software in a sandbox so that even if the browser gets hacked your computer won't be affected outside of the sandbox.

But instead of using shitty old firefox releases, I would rather recommend a different non shitty browser (like palemoon, which is basically firefox pre-cucked version) and install the last flash player (links on archive.org) and finally swap the player plugin with the one swfchan provides.

There have been literally hundreds of guides and threads how 2 do this over the years, so I'm not gonna repeat all the stuff like mm.cfg and mms.cfg.

Voilà, get your flash back and your freedom.

Remember, choosing to use Firefox/Chrome/Edge is your own fault and you do deserve shitty ruffle "emulation" for that behavior.