User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:36.0) Gecko/20100101 Firefox/36.0 Build ID: 20150320202338 Steps to reproduce: Downloaded a file Actual results: The download manager lists the file as 128 MB, but Mac OS X says the file is 134 MB. The actual file size is 134,579,600 bytes. Expected results: Firefox should use the standard units and match the operating system, and list the file as 134 MB. It looks like this issue was visited many years ago. Now that operating systems are moving to the standard units, firefox should follow to avoid causing further confusion.
see the long discussion in bug 106618. I think sticking to the default operating system behavior has some merit though, Windows uses MB intending MiB, while OS X doesn't (MB is MB). I'd not be sure about Linux. The most problematic thing here is the user suprise of finding a different file size in the system file manager.
It looks from bug 106618 that the issue was touched briefly in 2008, but not much for the last 10 years. The situation has changed since then. Linux has generally also adopted the standards (see for example https://wiki.ubuntu.com/UnitsPolicy and http://man7.org/linux/man-pages/man7/units.7.html ). Firefox should also adopt the standard units, though I'm not sure whether it's better to have the Windows build have an exception to behave differently.