Firefox does not stack tabs like Chrome does and does not load all the restored session tabs at once either. Aren't these dumb design decisions enough to switch to a product where people actually think about how it's going to be used?
As someone who always has too many tabs open, I consider this a plus.
I do however think you can disable this in about:config : browser.sessionstore.restore_on_demand
For what it's worth, Chrome stopped doing this for me in a recent update (I think version 71).