There have been a few bad signs over the years.

Most striking was the "Looking Glass" plugin. This was a Mr. Robot (popular TV show at the time) promotional plugin that would alter the behavior of a few tie-in websites as part of an ARG. Besides that it was "harmless", though had a vague description rather than saying what it was.

It was pushed by default to users using their user study framework. It was launched quietly enough and without going through the normal process. Even a lot of firefox devs didn't realize it until the press blew up.

And one of the responses to the push back was:

we heard from some of our users that the experience we created caused confusion

Despite Firefox leadership and marketing being the ones who were confused about the proper way to use their own user study framework, or avoid launching bad changes.

Aside: Mozilla also only just stopped accepting cryptocurrency donations in 2022, despite ostensibly caring about the environment and the internet.

Overall Firefox is still pretty good, despite being under-invested in by Mozilla, but if you use it you should recommend that at the end of the day there's a lot of corporate influence in it right now.