I found this by mistake, but it’s pretty neat. In Firefox, you can do tabbed browsing. Each tab can have a separate page in it, but they all show up in the same window. This makes it easy to visit multiple sites at once and switch between them without having dozens of different windows open.
If you have two or more tabs open, though, you can set them ALL to be your home pages. Click on Tools, and then Options. On the general tab, where you can set your Home Page, the button that usually says "Use Current Page" has changed to "Use Current PageS." Click this, and then Ok.
When you start Firefox, now it’ll open multiple tabs with the pages that you specified. So you could, for example, open the school’s portal page in one tab and your personal Protopage in another.