Major issues:
1. Lack of multilingual spell checking (without having to manually switch languages). For non English natives this is a must since we write in at least 2 languages every day. There is an extension that tries to solve this but it is quite slow to the point that I've had to disable it.
2. It just feels slower than Chromium browsers and Safari, at least on macOS. In Windows it's blazing fast but I have a Ryzen 3700X there.
3. On my MBP I get random CPU spikes from time to time and I'm forced to close and reopen FF. I haven't seen this in my iMac or Windows machine. I've never experienced this with any other browser.
4. On Android FF seems to be a battery hog. I haven't done any serious measuring but since I switched to FF my battery has gone from 3-4 days between charges to 2-3 days.
Minor issues:
1. I think the UI is inferior to all other browsers
2. I've had sync issues. I try to login to a website and for some reason FF will not write the credentials unless I manually sync. This has happened in iOS and Android.
3. I've had several minor issues on iOS. For example yesterday there was a weird bug in Twitter with the keyboard suggestions. I know it uses WKWebView under the hood but Chrome for iOS does not have those issues.
4. For some reason Mozilla decided to remove an extension that I use very frequently called UI Stack.
I don't think major 2 is objectively true. Firefox is faster than Chrome in every benchmark test I've seen for two years now.
As for minor 4, the blame is with developer. The API that was eliminated was EOLed long before they removed it, and it looks like he's doing some security violation https://github.com/gigobyte/ui-stack/issues/6