TLDR: Firefox is not implementing new web standards
Firefox is a great browser, but it's not implementing new web standards.
I want to use WebUSB and WebSerial to flash devices, but Firefox won't let me.
I want to use Speculation Rules to reduce latency, but Firefox won't let me.
Because of Firefox not supporting new web standards, users will switch to the much worse chromium.
Here is an incomplete list of things that are missing from Firefox that are in Chromium:
WebUSB and WebSerial (the reason it is missing is a alleged security risk for people who ignore warnings)
Speculation Rules
Certain referer policies
User-Agent Client Hints API
WebGPU (outside of Microsoft Losedows)
Here is an incomplete list of things missing in Firefox that are not in Chromium ranked by usefulness: