They killed Weave (aka Sync 1.0; which was somewhat weird but simple enough to comprehend, reimplement and self-host), replacing it with an NIH-reeking over-engineered abomination that's the very antithesis of standard, open or public. Most people just ignored it as "that's Mozilla own infrastructure, they don't have to make it open, design it well, think of others, or anything else". I could not.
They tried to push a fundamentally flawed Persona/BrowserID standard that continued the trend to remove users from their "own" identities while claiming it's a pro-user pro-privacy move. I can see the logic, but I'm of firm opinion that it would've done more harm than good. I'm glad the project died without gaining any traction and WebAuthn (which has its issues, but where users are the source of their identities) took over. That's what BrowserID should've been, but Mozilla just went with the flow and refused or failed to fight for identity ownership.
It's things like those what made me regret using Firefox (but again, everything else is worse), not some home page sponsored links. That's where they stopped to differ from the rest for me. Mozilla used to be a beacon of doing things right even if it was challenging, fighting for a better web. And they became just another software company, that put their glorious past on all the ads (how they're so pro-everything good) while failing to live up to those high standards.
They had an user agent, but they butchered it and made it just a browser.