Normally I'd say "you must be kidding" but I've learned that people here often don't know stuff that seems very obvious.
The big change for millions of us was Firefox Quantum, which came out in November 2017.
https://wiki.mozilla.org/Quantum
Mozilla is proud of this because it involves some Rust stuff, but it was a massive loss for the extensions world. It removed (or disabled) XUL, Netscape's extensions language:
https://en.wikipedia.org/wiki/XUL
To quote from the Classic Addons Archive:
«
This catalog contains 93,598 versions of 19,450 Firefox add-ons created by 14,274 developers over the past 15 years using XUL/XPCOM technology before Mozilla decided to ruin the classic extensions ecosystem and go exclusively to WebExtensions.
»
https://github.com/JustOff/ca-archive
For me, before Firefox 57, it worked superbly for me. Afterwards, of my 13 essential addons, only 2 still worked and one of those was trivial and unimportant.
It destroyed my browser and drove me away from using Firefox after nearly 20 years, dozens of computers and ½ dozen different operating systems.
Other cut functionality:
The best cross-platform FOSS email client, by far, is Mozilla Thunderbird. Mozilla has repeatedly tried to kill it.
https://www.theregister.com/2020/01/30/mozilla_thunderbird_h...
Mozilla also has cut off Seamonkey, the original Mozilla Internet Suite.
https://www.seamonkey-project.org/
Mozilla code is the one remaining FOSS WYSIWYG HTML editor, as included in Seamonkey.
It threw it out.
So far the following forks tried to maintain it:
Kompozer: https://sourceforge.net/projects/kompozer/
NVu: http://www.nvu.com/
BlueGriffon: http://bluegriffon.org/
Mozilla has no real idea why people use Firefox or what they want. It has forced wildly unpopular new features on people, such as the Australis theme.
https://support.mozilla.org/en-US/questions/974802
It's forced telemetry that kicked the entire browser offline for millions:
https://www.theregister.com/2022/01/18/foxstuck_firefox_brow...
And yet it includes a huge complicated "developer mode" that 99.9% of users never want and never see.
Mozilla's Boot2Gecko became FirefoxOS which became KaiOS, the fastest-growing mobile OS in the world.
https://www.kaiostech.com/
Mozilla abandoned FirefoxOS.
KaiOS is proprietary. Mozilla licensing should make that illegal. Is Mozilla enforcing it? No. Who wrote the license? Mozilla CEO Mitch Baker.
Salary: $3 million in 2020.
Also 2020: Mozilla fired 250 people, quarter of its workforce.
https://www.theregister.com/2020/08/11/mozilla_staff_layoffs...
Summary – cut functionality from Mozilla products this century:
• XUL extensions
• Email, both integrated and standalone client
• Newsgroup reading
• FTP support
• XMPP Chat
• Web editing
• Native FOSS mobile OS
• Multiple themes.
And so on.