They can! And, as you note, Mike Hommey is moving towards doing so for Debian, at least for our unit tests (getting reliable perf data would be involved at the minimum you'd need the same machines we have running our perf tests. But it's doable!). And I'm sure that passing all our tests would give the folks involved in making the trademark decisions a lot of faith in the changes distributions have made!
But without that, and a commitment to keep running the tests (when you upgrade a library from under Mozilla, you have to make sure you keep things working!), justifying using an external library is a lot harder.