This might be caused by an extension overriding your choice, or by external software that changes your Firefox settings.

(1) Try disabling ALL nonessential or unrecognized extensions on the Add-ons page (don't limit your search to official Yahoo add-ons):

Tools menu > Add-ons > on the left side click Extensions category

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

(2) External software may create a user.js file. Firefox reads this file at each startup and uses it to modify your settings. To investigate this possibility, check out this article: How to fix preferences that won't save.

And then finally you should be able to set your home page without interference and have it stick. Any luck?

The only thing that worked for me was to go to about:config, Find the Preference Name..browser.startup.homepage, Dougle click on Value and type in your homepage starting with http://