I'd like to use FireFox Beta to see some of the new features. I'm OK with occassional stability issues but I'm concerned about security. For example, if there's a vulnerability will Mozilla necessarily be as responsive to correct it in the beta cycle as they would with a stable release?
The possibility that there is a well known exploit for an old version of a piece of software is much higher than for a recent version. But you cannot make this relation to the beta of the software.
But concerning that there are only a few person using the software the profit of an exploit is much higher for a recent or old version than it is for the beta. (unless the bug is undetected and gets merged into the stable branch). Of course the beta software changes more often than the stable release, which makes it even more unattractive to design exploits for.
You have to make the decision on your own what you consider to be safe (and usable), and especially don't rely on it as your only concept of security but combine it with other - far more useful - security measures like script blockers etc.
I personally use aurora (the nightly version) under windows7 and use the unstable firefox build within gentoo (linux). Because I personally think that this might aid my line of defense, but I would not vouch for that nor recommend it.
Edit: Of course it is highly dependent on your definition of security, as you can read above for me this is mainly malware and scripts that invade my system or privacy. But a piece of beta software might be able to do nasty stuff on its own (eg delete files on your drive), which isn't much of a concern for me, so again, you have to decide for yourself.
Firefox's beta can for most purposes be considered a release candidate build - it's mostly frozen, and any security reviews for code are generally done before the merge to beta. It does contain newer features that haven't been as broadly tested as they would be by releasing the software, and it may also already contain security fixes that haven't shipped in a release yet. There's really not a perfect answer here, because the security "footprint" of the beta is different from the release in both positive and negative ways.
I'll go out on a limb though and say that the security for a Firefox beta is probably better than other products labeled as beta, and that's out of some familiarity with the Firefox development process. Betas are also going to be less likely to be targeted for exploits - - the odds are someone who finds an exploit during beta period will either report it, or if they have malicious intent, they will hold on to it until the bug ships in a released product so that they can exploit many more users at once.