Is it worth to learn Foundation too after learning Bootstrap?

I think it's worth mastering Bootstrap, but it's good to keep your finger in all the pies! Most frameworks are really similar anyway, so if you know one it's likely you'll pick up another really easily. They just have slightly different ways of doing things, slightly different class names etc.

Some frameworks suit different sites. I prefer Bootstrap, but am currently using a framework called Materialize for a website that needs cleanliness and simplicity. It just depends I think on the project! Foundation uses Flexbox, which doesn't support older browsers... and has a smaller support network then Bootstrap, but it's cutting edge and super quick and focuses heavily on mobile first.

So, I'd master bootstrap, and then use other frameworks based on your framework knowledge as and when you may need to! :)

The first time I looked at any other framework than bootstrap, i already knew the framework I have looked at.

Frameworks like stated above are similar. In terms of CSS frameworks, they are build on the same concept, so really, if you learn one framework, then you can master any framework with few days. You need to check out their docs, and read all of it so you know what they offer, spend a day doing or reading their docs, nothing else.

Well, unless some frameworks have Sass and stuff, whcih i see that foundation is more on Sass, then ud might learn Sass really.