Here's a feature I didn't know existed. Type the following in your address bar in Firefox:
about:performance
You'll get a nice, real-time readout of what's slowing up your Firefox by eating system resources.
For some reason https://dev.to/ was eating my system resources using 100% of a CPU core just being open. Switching to running Firefox Nightly solved the problem though (no idea why).
Any takers on why Firefox and Firefox Nightly behave differently with respect to CPU usage on the dev.to site?