The problem doesn't sound related to anything "new" in Firefox 2.0.0.15 or 3, it's something with the installation on your system.
So, I suggest you do things ONE step at a time and post the EXACT messages you receive in this thread. Paraphrasing isn't good enough but if you take a screenshot of or
write down a message you receive and post it here, we can get all the info we need and you won't have to worry about missing anything that might be important.
First, is Firefox 2.0.0.14 running on your system now? If so, do you have any extensions installed? If you do, please list which extensions you have installed.
Second, if Firefox 2.0.0.14 is running on your system, empty the browser cache (click "Tools", then "Clear private data", and make sure "Cache" is checked). Once the cache is emptied, check for updates to pull down a "fresh" 2.0.0.15 update. To do this, click "Help", then "Check for Updates".
Before installing the 2.0.0.15 update, please post the answers to the questions above. Then, we will proceed based on your answers.
Lastly, what protective software do you have installed and running right now? Please post the application name and version, like ZoneAlarm 4.0.23, Ad-Aware 2008, etc.
Thanks!
Peace...