How can I set default homepage in FF and Chrome via javascript?
You can't do it in FF because of security. Check out this article. Your user would have to change the signed.applets.codebase_principal_support setting to false. Probably not something that is worth counting on.
Setting current page as homepage
I asume you are not asking how, you as the user of the browser, to set the homepage; this might be useful: https://stackoverflow.com/a/10751061/2612112.
universal javascript for set homepage functionality
This is an earlier discussion thread on the subject: How can I set default homepage in FF and Chrome via javascript?
Basically, there's no universal support and changing the homepage is generally considered a very annoying thing to do.
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