You probably already have checked the Options dialog to make sure Firefox is still set to start up with your home page. In case you haven't, this article has the steps:

• Startup, home page, tabs, and download settings

Sometimes and add-on or external program will create a user.js file in your Firefox settings (profile) folder, and Firefox will use this file to override your previous session settings. To track down and remove a user.js file, please see the steps in this article: How to fix preferences that won't save.

Note: By default, Windows 7 hides the .js file extension. To unhide it, so you can work with file names as accurately as possible, please see this Microsoft support article: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

Any luck?

I should mention two other possibilities:

(1) Crash recovery: if Firefox believes that it crashed at its last shutdown, it will automatically restore the last session.

(2) Advanced SystemCare and other "browser protection"/"home page protection" utilities: these programs may roll back your browser settings changes.

You can check if you have a user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

• http://kb.mozillazine.org/browser.sessionstore.resume_session_once

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Preferences_not_saved