@killyourfm Thunderbird is great. Nuff said really. My desktop client on all machines now and K-9 on the Android phone. The only thing that isn't super fast is where I still have to access Gmail 'because legacy'... but not for much longer. IMAP into my own mail server accounts is lightning fast but the g-behemoth... soon be gone!
Firefox is still working well for me but I recall when I ditched it some years ago, after it contracted 'galloping featuritis' and ground to a snail's pace 😕