My main source is Firefox update release notes linking to the release developer notes, which mention changes with links to further documentation. (example: 142)
Which is why the most recent Firefox update splash screen no longer containing a link to the release notes irritated me quite a bit. I hope they revert that. If it's now only ad-space for features unrelated to the release then I'll have to look into how to disable it alltogether.
Other than that, communities like this one where people post links is quite helpful.