If you already upgraded to Firefox 14, upgrading to 15 is painless. Download the new MSI, tweak it if you want, and push it out. Thanks to the improved extension handling in recent versions nothing else needs to be done.
Firefox 15 introduces silent updates, though it's disabled in the MSIs because FrontMotion rightfully assumes that if you're doing it this way you want to handle pushing them out yourself. You don't have to do anything to turn it off.
Using silent updates would mean you wouldn't have to worry about keeping up with each new version of Firefox, you can just install it on your image and it will take care of updating itself, like Chrome. It's also beneficial in that new versions will be applied whenever Firefox restarts and not the entire computer. However, you'd lose the ability to roll out updates in a controlled manner and to do compatibility testing beforehand. I'm going to stick with the MSIs for now, but if I didn't have to rely on extensions for things like IE-specific sites I would consider this.
Firefox also has extended support releases if you'd prefer the upgrade treadmill not move so fast. FrontMotion provides MSIs for them too. I'm sticking with the standard releases for now because I wanted all the memory use improvements that have been landing in recent versions, and the improved extension handling has made upgrading less of a burden. However, switching to ESR would mean you would only face a major upgrade once a year while still getting security fixes, so it might be an attractive option for some of you.
Although this is technically a blog, it's primary content is a series of articles on how to get Firefox working in a corporate Windows environment. Later ones build on earlier ones, so you might want to use the Table of Contents on the right to read through it chronologically instead of reading straight down from here.