My experience on Linux running on very old hardware (4th Gen Intel) is also good. Firefox feels quick and snappy. It uses a reasonable amount of resources, and has a relatively modest memory footprint by modern browser standards. In comparison, Chromium makes my fans spin on every site and eats several GB of memory.

The annoying part of Firefox is that development seems a bit stagnant in some areas, especially taking into consideration the amount of resources Mozilla has. For example, bookmarks and history still rely on a very old native UI that is quite clunky. Customization via user.js is too imperative and most options are largely undocumented.