I'm not sure what you mean by "never activates." Can you try the steps in How to download and install Firefox on Mac and if that doesn't work, try to clarify what doesn't happen on your computer?

Hi, thanks.

Basically, the upgrade, step 1, installs immediately. Step 2, which says to drag the firefox icon into the 'A', when prompted, never prompts. I've waited up to 10 minutes so it never completes the install.

Ah - you have to double-click the disc image icon in the downloads window first. Then it will open that up and you should see the Firefox icon that you can drag to the application folder. Here's a video of how it should work http://people.mozilla.org/~mverdi/video/mac-install.theora.ogv