could it be related to this Known Issue in firefox?
We have made some changes to fix security problems involving one site targeting links and form submissions into a frameset created by another site, however these changes may cause problems with some sites. If you encounter problems where sites are trying to use this capability legitimately and you need them to work, you can use about:config and set docshell.frameloadcheck.disabled to true to disable the security check. Please file bugs on the URLs that are problematic, if you encounter any.
When I load the adsense login page, it looks like that part of the page loads separately, as if in an iframe.
That could mean it is somehow related to the security fix from firefox?