There are no more security updates for v91 ESR after this month, so beware. You could try using SeaMonkey or Waterfox if you value an older Firefox UI interface, but they are usually even slower to backport security fixes. For example, SeaMonkey just backported fixes from v91.11 ESR, which is older than v91.12.
I've found that current versions of Firefox are quite good with memory management. I've got 130 tabs open at the moment (I'm a tab hoarder) and it's using less than 750MB RAM right now. (I'm also using the Auto Tab Discard addon, which helps a bit). You might want to debug your Firefox profile and disable unneeded addons or start a fresh profile. A fresh profile usually does wonders.
For inches to metric, see https://support.mozilla.org/en-US/quest ... wer-870648.
For date, maybe try altering some printer about:config values. See https://support.mozilla.org/en-US/questions/1117290. For date alone, this is the closest you can get: https://support.mozilla.org/en-US/questions/1128476