Which version of FF are you using? I just downloaded Firefox 59 and the Trojan.OSX.Flashback was apparently downloaded with it—or within it, actually. I was alerted to it (by ClamXav Sentry) as Firefox was being installed. It's more likely Firefox is the issue, that the trojan horse—or false-positive code—was installed with it… and yes, I downloaded it from Mozilla's site, not some dodgy third-party software site.
If you only have the issue in FF, it's probably not Flash—this time—or you'd have it in Safari, too. If you've recently downloaded FF, or upgraded an older version, I'd check your backup(s), go back to an older version of Firefox, and then scan your system again to be sure it's not there. Then install some form of virus/trojan horse checker that checks all of the time, as you work, download files, and install software, etc.