Open up Mozilla Firefox if you haven't already.
Click on TOOLS on the menu bar at the top of the Firefox screen, then click on OPTIONS
see fig 1.1 below for a screenshot:
After clicking options you will see the Firefox Options dialogue box as shown below in fig 1.2
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Type the address of the webpage that you would like to use as your new homepage into the box provided (see Fig 1.2 above) and click the OK button.
If it is the webpage you are currently viewing just click the Use Current Page button then simply click on the OK button.
Notice in Firefox you can also use one of your existing bookmarks as your new homepage.