I read the descriptions for the Dcentraleyes and LocalCDN addons which cache popular JS frameworks and page assets to enhance privacy and speed up pages that use them (since the assets were downloaded beforehand). Does Firefox have any built-in functionality to cache frequently used assets, or are there any addons that do so?
For instance, If I access Reddit.com a lot, I would want all the resources that all those Reddit pages have in common to be cached automatically to make loading pages from the domain faster.
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).