If the decision has been made to ditch the GPL, why not move to the Apache 2.0 license rather than to the MPL? It seems like this would be a lot less work for everyone, because it would allow LibreOffice and OpenOffice to share code.
I honestly do not see that much difference between the MPL and Apache 2.0. Sure, the MPL requires you to release the source code to files that are MPL'ed. But if you want to keep things proprietary, you can just move that functionality into another file and keep that file proprietary. Rinse and repeat as necessary. The MPL doesn't force you to release a single line of code, except for the boilerplate required to call a function in a different file. What's the point?