Firefox difficulty: I've had a problem pop up a couple of times that might interest firefox devotees (they might discuss it on the Mozilla forum too but I haven't bothered looking). I've had my firefox "profiles" get wiped a couple of times after a system crash. It seems that, if FF is running and you experience a BSOD or a major crash for some reason, the FF profiles can get corrupted or wiped out. That means you lose bookmarks, passwords, and add-ons - sometimes. I think only one file really gets wiped out, but you either have to recreate that file or start over.
So... I make a habit of backing up my firefox profiles occasionally. They're usually located in C:\Documents and Settings\<your windows name>\Application Data\Mozilla\Firefox
In theory this "problem" would crop up in Flock or any other FF-derived browser, AND Thunderbird.