I'm not sure whether it's the same on .com.au as .com, but if you are logged in to your Google account, Google will redirect you to the secure version of the site. If you are logged out of your account, you should get the page you request. (This may require using Google's logout link or clearing your Google cookies.)

Note that typing in the URL could behave differently because Firefox's URL-bar-Autofill feature will prefer HTTPS or HTTP when it finds both in your history.

If you stick with the secure Google URL, you might skip the step of carefully editing the URL when you want to load another site. By default, clicking the URL should select all of it so anything you type will replace it. Typing www. is pretty fast, or if the site is in your history, typing the more unique part should call up the URL pretty quickly.

Hi, I don't have a Google account.

I do want to know why Firefox alters my home page address by changing it to a secure site?

Firefox doesn't change your home page from what you have set in your options. If Google isn't changing it, perhaps one of your add-ons is responsible?

You could try Firefox's Safe Mode. That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference when you click the Home button?