Did you know you can get Flash MSIs from Adobe? Well you can. Just head on over to this page to apply for a license, which doesn't cost anything.
Also good news is that you don't have to worry about one installing before the other. If you're pushing out Firefox and Flash via MSI and Flash happens to install first Firefox will pick it up just fine.
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.