Thanks guys.
I just updated Flash by running the install/uninstall that Gopher John linked to (2.94-meg file for FF). It worked fine without having to uninstall Flash manually first. I DID NOT have to shutdown avast — and never had to shut down avast in the past for any software installation.
I think one time a couple of years ago I had trouble updating Flash, but that was because I tried running it rather than downloading it. I run FF under 'drop-my-rights' which protects some directories from being written to.
Thanks again.