Not just you. Firefox is my main browser, but for a growing number of JS-heavy sites I've been using Safari instead. These are not necessarily ad-ridden either, but rather interactive apps (Jira, Whimsical, etc.) were FF is getting super laggy and it's a real distraction. Sad how much faster Chrome and Safari behave on them :/. This is on a new 2020 MBP with plenty of RAM.