Hi DavidR.
I have beta tested the Flock developer's preview now from day 1. And I test security on Flock. Never crashed on me even once, FF dozens of times, especially when you open it to view an update page (ClamWin version, from ZoneAlarm), slow when you use the full format.
The add-on Fasterfox helps considerably, but Flock is the more stable. leaner, faster browser with the better user-friendliness. And that what it is all about. I think the consistency of the coding bunch of Flock is greater, the noses stood one way, and the results proof it. Both browsers would not bring the difference, nor FF nor Flock, was n't it for the add-ons like developer toolbar, NoScript, Adblock extra, Dr. Web hyperlink scanner etc. These things really make these browsers shine.
FF does strange things now and again, and not in all cases these things are reproducible. For the not so fainthearted have Flock next to it, and give it a try, or wait one month and a half or two for the official launch of the full version.
Yours truly,
polonus