I don't have enough experience and knowledge of the LastPass folks to answer your questions. The dropbox issue is a non-issue if the data stored on the site was encrypted well before it ever was sent off your computer.
In general, I have different criteria for selecting software that is critical for me. Some random plug in, pictures, a cool app that doesn't store say, banking information. I'm all about giving people the benefit of the doubt.
My core operating system and my password generator/manager isn't time for that sort of experimentation. I am very happy using 1Password and understand their business model. Yes, for a time it was free, but it was obvious that they had a sustainable and reasonable pricing model. Now that they have (I hope) millions of users, they can charge a modest cost to ensure they have great response, great support and most importantly, great engineering. I will make more allowances for initial downsides when I believe more in the long term decisions of the company making my core tech and less likely to dabble in newcomer efforts (no matter how worthy they seem).
I would evaluate whatever software you choose by trying LastPass, 1Password and several others with fake data, but not trust your critical information to software until you are comfortable knowing who made it, how they secure data, and that they won't likely sell out to a low bidder at the first wave of $25k. The value of millions of people to add to a corporate mailing list is attractive to owners that don't have an otherwise viable business model. I try hard not to be part of their short sell-off strategy by giving them my most valuable information for free.
I hope this helps you frame your decision and someone else that has gotten comfortable with LastPass weighs in with their take on your questions. It's a good one and high time more users start holding their authentication credentials a little closer and more securely between sites.
In short: You can't. Firefox always uses it's own store for passwords and will never access the Mac OS X keychain at all. You either have to manually copy your credentials into Torbrowser/Firefox windows or use a third party software like 1Password that integrates across browsers.
Be aware that if you use a login on a website that hasn't been created over tor there is a good chance you're giving up your anonymity by yourself. Tor cannot protect you from posting your identity (by logging in with a username and password) by yourself!
Best Answer
Perhaps you mean 1Password by AgileBits? If so I can answer that as a happy user of 1Password that you can't. Migrating or syncing passwords from/with the keychain is as far as I know, not strictly possible.
But 1Password (at least) is designed as a replacement for the Apple Keychain. the migration is not without its pain points (manually moving passwords over wholesale, or a few at a time as they pop up is not fun) but once you have all your passwords moved over your need for Keychain is gone.
Also, you can get an iOS version of 1Password as well which syncs with the desktop versions (either through the cloud or locally within your WLAN) and ties in nicely with Mobile Safari and other apps on your iDevice.