All code in active projects which is currently under the MPL (perhaps in combination with other licenses).
There are two projects within Mozilla which have historically had greater independence - Bugzilla and Rhino. Neither uses the usual tri-licensing scheme - Bugzilla is MPL 1.1 only, and Rhino is MPL/GPL. For those projects, whether to shift or not is a discussion to be had with their leadership([https://bugzilla.mozilla.org/show_bug.cgi?id=680131 Bugzilla discussion]).
Should an upgrade be agreed, any project which is MPL-only (such as Bugzilla) would need to add the Exhibit-B GPL-incompatibility language to their licence header.