BB 41065: Hardcode the limit of storage quota.

Currently, we return min(10GB, 0.2 * available space in the fs).
This is bad for systems that have less than 50GB free, especially live
systems, such as Tails.
Set a pref to replace the available space with a hardcoded limit
(5GiB).

XHR requests initiated by extensions are exempt from RFP.
Therefore, they report the actual app name, instead of Firefox, and the
actual Firefox minor version.

This happens whenever the app name has been customized and does not
match a hardcoded "Firefox".

BB 43236: Disable vsync on Wayland.

Vsync leaks the monitor refresh rate, therefore it is already disabled
on other platforms, including Linux/X11 and Windows.