I can't speak to why they upgraded without asking you. However, 3.6 has been end-of-lifed and will no longer receive security updates. You'll be much better off using the Extended Support Release.
Mozilla doesn't make it easy to find the ESR download page. It's here:
You will not be able to install multiple versions of Firefox side by side. You'll need to use Firefox Portable Edition if you intend to have multiple instances.
Mozilla doesn't make it easy to find the ESR download page. It's here:
https://www.mozilla.org/en-US/firefox/organizations/all.html