I don't know if you can just turn it off. I know you can uninstall just the Flash Player, and then when your done testing, you can reinstall it. That seems a little crazy to me though, maybe someone else will have a better solution. I would like to know if theirs a better way too because I too am working with Flash and Dreamweaver all the time. This info could help out a lot.
Couldn't you just move the plugin out of the folder /Library/Internet PlugIns/ (not sure on exact name of this folder; not in front of my Mac currently) then restart Safari/Firefox?
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The flash plug-in is located under library/Internet Plug-Ins just move the files associated with any plug-in and restart your browser.