Note: At this point mainly a TL;DR for [1] and [2] given a system with a solid state drive (SSD) and plenty of random access memory (RAM). To be extended.

Firefox RAM cache

To make firefox cache exclusively in memory, add the following firefox settings (either from about:config or user.js) [^ram][1]

[^ram]: Note: these settings are for a 32GB RAM system, replace the 32.0 as appropriate.

browser.cache.disk.enable = false
browser.cache.memory.enable = true
browser.cache.memory.capacity = 41297 - (41606 / (1 + ((32.0 / 1.16) ^ 0.75)))

Firefox RAM profile

If you are questioning, “can we go the extra mile?”, the answer is yes. We can do the same not only for cache, but for the entire firefox profile.

Without going into the details (see https://wiki.archlinux.org/title/Profile-sync-daemon for those) on an Arch Linux system —after closing firefox— we do so as follows:

sudo pacman -S profile-sync-daemon
systemctl --user enable --now psd
psd preview

Having both enabled and started (--now) the service, we see that the service and timer are now active.

rofile-sync-daemon v6.48

 systemd service: active
 resync-timer:    active
 sync on sleep:   disabled
 use overlayfs:   disabled

However, as is inherent to all, there is a trade-off involved. We traded persistence for speed that came with increased volatility. That is, we now lose our browser state should firefox or our system crash. Its remains either swept away for space reclaimed or out in the ether as voltage seeps out of our RAM. The good news is that psd already mitigates this for the most part by periodically syncing the profile (now in RAM) to disk.