For a long time Firefox Desktop development has supported both Mercurial and Git users. This dual SCM requirement places a significant burden on teams which are already stretched thin in parts. We have made the decision to move Firefox development to Git....
I’m only a native English speaker, so guess I could be interpreting it wrong.
You should try being a native English reader.
What it means is “they will not be accepting pull requests at this time.” Whether or not they are open to changing this in the future is not specified. They have not specifically stated that this is off the table, nor have they stated this is their intent.
Firefox Development Is Moving From Mercurial To Git (groups.google.com)
For a long time Firefox Desktop development has supported both Mercurial and Git users. This dual SCM requirement places a significant burden on teams which are already stretched thin in parts. We have made the decision to move Firefox development to Git....