Old Flash

I’ve been wondering for some time if old versions of Flash on a computer are a vulnerability or not.
Today while looking into the vulnerability of Flash for Mozilla, I found an article from Adobe which states:

“For Internet Explorer, only one version of Flash Player can be registered for use at any time. Older files can be removed, but this is not required as part of the update.”

So that solves one mystery but I’m left with the one I was originally researching.
Flash uses a separate install for Mozilla and Opera. Those files get installed to the browser’s plugins directory. Although I have the latest version of Flash for IE installed, when I run a version test from my Firefox browser, I find that it is running an old version.
This makes me worry that the Firefox users may remain vulnerable to any Flash vulnerabilities that are not IE specific.