SPAXX on fidelity is pretty easy to get into. Just open a Fidelity account and xfer your money there. During high interest it was getting 5% now its about 4% I think. You get paid out every month.
To invest I think it makes more sense if you are holding for 5 years minimum
Using a firefox derivative I dont think is a good option as it will always be behind on security updates.... I guess I am going to wait until the Orion Beta / software comes to Linux which was announced recently. Orion is a WebKit based browser that is on iphone / mac