Gmail is really crazy slow on Firefox, it’s such a deal breaker. I have regularly 30s to load the web application, it’s closer to 2-5s on Edge with the same setup (same ad blockers, etc).
That has to be something wrong with your PC, it only takes a few seconds on even my 6 year old laptop on Firefox to load up from zero. Switching between folders is under a second as is opening an email. Firefox (latest Ubuntu)
I suspect this is because chrome supports WebSQL while Firefox does not.