Why not just use FF container tabs to keep that separation?I think it's always good to have a backup browser. Not even necessarily for sites with compatibility issues (which is fewer and fewer) but to have a separate browser for certain tasks. I do all of my banking and sensitive stuff in Firefox and Safari for all of my other browsing.
That said, I do use Safari sometimes for non-sensitive things when my privacy settings and extensions get in the way of stuff like loading the reviews when shopping.