It does this by default.
Firefox
A place to discuss the news and latest developments on the open-source browser Firefox
Yes, every browser caches resources that multiple pages of the same site use, unless the site instructs them not too.
It is also the case that almost every modern browser does not share those caches between different websites, to avoid providing a mechanism for them to share data. This means that for websites, it is no longer beneficial to use CDNs, if it ever was - in practice, it was also the case that only very few CDN resources were actually shared between different websites (since they all depended on different versions or different CDNs).
Yep, every browser has a browser cache, I believe in Firefox it's set to 1GB by default? It will cache assets like JS or images on the same domain name.
Firefox changes the capacity dynamically, I set browser.cache.disk.capacity to false in about:config and browser.cache.disk.capacity to 1024000 (the storage amount in MBs)
Gotcha, mine always seems to be 1GB of space usage on all my PCs so I figured that was just the default.