OK, but the developers of Mozilla are specialists, too. Why not trust them? Or is nnethercote part of the Mozilla dev team? MemShrink will hopefully just be merged into the core?

I'm a Mozilla employee and started MemShrink. I also write about it regularly: http://blog.mozilla.com/nnethercote/category/memshrink/.

MemShrink has been running since June 2011 and there have been many improvements in Firefox's memory consumption since then; these have shown up in Firefox 7, 8, 9, 10 and 11.

He's part of the Mozilla dev team. Memshrink isn't a branch that's waiting to get merged, but an ongoing program to reduce overall memory usage.