Because while you do have control (and “copies”) of the source code repository, that’s not really true for the ecosystem around it - tickets, pull requests, …
The announcement to drop Mercurial quite clearly states that their workflow won't change and that GitHub pull requests are not considered a part of their workflow.
Also, that's entirely irrelevant to start with. Either you care about software freedom and software quality, or you don't. If you care about software freedom you care about having free and unrestricted access to FLOSS projects such as Firefox, which GitHub clearly provides. If you care about software quality you'd care about the Firefox team picking the absolute best tools for the job that they themselves picked.
The announcement to drop Mercurial quite clearly states that their workflow won't change and that GitHub pull requests are not considered a part of their workflow.
Also, that's entirely irrelevant to start with. Either you care about software freedom and software quality, or you don't. If you care about software freedom you care about having free and unrestricted access to FLOSS projects such as Firefox, which GitHub clearly provides. If you care about software quality you'd care about the Firefox team picking the absolute best tools for the job that they themselves picked.