The set of telemetry collected by default depends on the build channel: release builds collect much less data than nightly or beta.
You can also see exactly what data Firefox engineers can see at https://telemetry.mozilla.org/ -- there is no special behind-a-login view for this stuff that I know of.
Disclaimer: I work on Firefox.
They all include expiry dates, and probes with perpetual lifetimes require devs to take responsibility for them.
All probes need to go through the data collection process outlined at [3].
[1] https://searchfox.org/mozilla-central/source/toolkit/compone...
[2] https://searchfox.org/mozilla-central/source/toolkit/compone...
Every telemetry probe requires a privacy review in addition to the code review. If you want more, the source code is available online. Do you have more in mind?