Mx25 Infinity on my 2008 hp G62 laptop. (Whoops, using xfce with gnome fork). Acting as a kodi server.
antiX26 on my i7 laptop. zzzICE wm. Daily driver.
PIOS on my pi3b. Xfce, jellyfin server.
All idle at less than 800mb. The jellyfin server doesn't have to do transcoding, serves up three streams at a time.
The kodi box is also the backup dev box, but with 8G of ram, I've never seen it at 50 percent.
The i7 is new as of 2 months ago. It's just shit fast compared to my usual. I'm still trying to catch up.
All of these Debian OS derivatives/compatibles I have run successfully and usefully from a 4G thumb drive.
I also ran Mx23.1 (wildflower) on the pi3b for about 8 months. Used it as my first kodi server. Functioned adequately if not spectacularly, and this is on 1G.
All run firefox as the default I believe.
Low memory options are out there and fun to use.