I use Fx 3.6.25 (native install) and, atm, Fx 9.01 from PortableApps.com
Just recently stumbled on the fact that with a simple change to an .ini file in the Portable (see the Readme inside it), one can run the native and the portable at the same time. So I can browse with both supported versions, Fx 3.6.x and 9.x, tile them side-by-side, and see how any issue, site, etc. displays on each, comparing in real time. Cool.
Of course, MZ tells us that 3.6.x will be put to rest in April 2012.
Even so, this could have multiple uses: Install Aurora, Nightly, whatever, on your HD, and use the latest stable release from PortableApps. Running them side-by-side could highlight any differences in appearance, behavior, etc., even for those who aren't troubleshooting an issue (or for those who troubleshoot issues officially

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Now that my lawyer is happy, does anyone think this is useful/worthwhile/interesting/garbage?