Well, I went and tried Chrome. ...It was a bit sluggish, but more consistently sluggish. Firefox will run MWA fine for a while but eventually slow down. Chrome is just slow from the get-go.

But, having learned of this new diabolical thing called "Hardware Acceleration" I went and disabled it, and MWA ran fine... at the cost of everything else I play on Chrome running slower.

...I'll have to do some more observations to figure out the long-term results.