The iPad 3 was slow to begin with.
As for Safari, complex JavaScript and media heavy websites are part of the issue. Even if Firefox was supported, it still wouldn't work well. Only reason Puffin works is because it's a proxy that does the heavy lifting server-side. On mostly text sites (e.g. fanfiction), Safari still works just fine on the iPad 3.
Things like reading sideloaded ebooks (non-DRM epub or mobi), comics (non-DRM cbz or cbr) and pdf should continue working forever with standalone reading apps that don't rely on servers. Same for watching offline/sideloaded videos.
At the very least, one can use it as a digital picture frame.