Is there an option to enable a 1 time flash plugin on Mac? Possibly with portable firefox or something like that?
I don't want to install Flash on my Mac, but I do find some legacy sites where this is required.
Google Chrome has Flash built-in, you could use that for those sites without having to install the stand-alone version.
Use http://sourceforge.net/projects/osxportableapps/files/Chromium/
This is based on @Anthony R answer, which mentions that Chrome has flash built in. Mixed with the concept of portable app nature (not leaving any traces on the Machine is it running on), this makes this a perfect setup to make sure Flash does not contaminate my Mac but still be able to use it when required.