Obviously it'll make the developer more efficient to spend more time twiddling his thumbs waiting for his code to compile rather than creating a simple build performance win that allows him to, you know, spend more time improving Firefox. Not to mention all the other developers who stand to benefit from faster builds.
Common Voice is part of the Mozilla Foundation, not the Mozilla Corporation responsible for shipping Firefox. I.e. what donations to Mozilla actually go towards.
Mozilla occasionally rolls out features in the current release via remote mechanisms. So it's rolling out to existing v101 installs now. I would assume that v102 will also ship with it on by default.
Also, they've specifically called that out in the advisory when they're aware of that being the case. See the last out-of-band security update they released for example:
Firefox only uses Freetype on Linux (the system-installed version) and on Android, and the vulnerable code path was only reachable in Firefox with a non-default pref set (gfx.downloadable_fonts.keep_color_bitmaps).