The manifesto exists since 2007, but they describe principles that Mozilla has held since it was founded in 1998[0]. I wouldn't personally describe them as a "politics first, software second" organization, but that's mostly because I think the distinction between "politics first, software second" and "software first, politics second" is pretty vague and useless. Perhaps you would describe them as such, and if so, they've probably been like that since the start, or at least since before the start of their market-share decline.

0. https://www.protocol.com/mozilla-plan-fix-internet-privacy