I have Portable FF 43.0.1 in my Win 10. Portableapps offered me to update it (Main Menu - Apps - Check for updates) to FF 45.0.1, quoting a size of 95Mb, which, just in case, I verified it was the size of the download after launching the update). On the other hand, if I launch the update directly from FF (Help - About - Check for updates) it downloads about 50Mb.
Why the difference?
PS: After updating from within FF, the Portableapps updater still shows that I need the update (at the same 95Mb). This is not surprising, but it could perhaps be spared.