Mysterious. I'm feeling for ya. Let's see, I'm atm on a 2011 Macbook, MacOs Sierra, using Mastodon on WaterFox, no issues*. I also use Linux Mint on an HP from 2008, Firefox, no issues*. Both on WiFi. My main station is an ancient hand-me-down iMac on a 100 foot ethernet cable, WaterFox on Sierra, runs really good.
*c.im has issues loading images on the laptops, mastodon.world has no problems with them.