I use uMatrix in Firefox to block javascript, cookies, media, etc by default. For any site I need to login, I can easily enable them.

I don't see it too often, but occasionally I do run across a site that won't load at all without cookies enabled. For these circumstances, I use Containers and Cookie AutoDelete.

The only situations where I tend to have problems are those where I have a third-party payment window that opens in a new tab. It sometimes takes some fiddling with the settings to make it work properly.

> many things does it break?

Almost nothing, surprisingly enough. Of course it breaks websites I sign into, so I simply whitelist those.