It is because by default Firefox automatic updating function has already set without asking for your permission.
Anyway, this is what you need to do to stop it from happening:
1. Click "Tools".
2. Click "Options".
3. A new page titled "Options" opens up as shown below.
5. It will show another page. Click on "Update" on top of the page.
6. Select (by clicking on the small circle) next to: "Check for updates, but let me choose whether to install them". (As shown in the image above)
7.That's all.
That's how you disabled or turn off Firefox from up automatic update its latest version.