Lion here too (10.7.3). Flash version 11,2,202,235 (bang up to date) and tried the same thing out on both Safari (Version 5.1.5 (7534.55.3) and Firefox (Version 12) with the same results:
Scrolling up/down fast improves the download speed and doesn't impact on the upload speed test results at all.
From (a hazy) memory, I never used the Flash test as it seemed totally erroneous anyhow (and this using my old Vista 32bit laptop).
My speeds for reference are normally 15778 Kbps down and 1111 Kbps up and are consistent using the Java based speed test.
The flash based test gave me 12 mbps down if mouse was idle and 16.2 mbps down if I went mad with the up/down scrolling. Interestingly, I couldn't improve on what is probably my actual connection throughput stats as I'm using ADSL2+ (Annex A) on a 24meg/1meg product from Be.
There was an online flash based game I once played, where I can just about recall, you could manipulate results in the same kind of way so I don't think it's a unique phenomenon to TBB. My own (uneducated) theory is it's got something to do with the graphics card. It'd be interesting to see if this issue is just Apple based or not (I suspect it isn't) and whether it's affected by the type and power of graphics card installed. Over to you Mr Bill!