This is a simple case of "it's not doing what you think its doing". Powertoys ruler measures how many pixels it physically takes on your screen; AFTER scaling. Scaling settings can be found under the System > Display > Scale & Layout.

You are probably on 150% scaling, hence you should get 48 x 150 / 100 = 72px. On Chrome the ruler will measure 2px less as it does not include the border but on Firefox the border is included.

On 100% scaling you will get the exact size of 48, at least on Firefox.

Chrome 150% Scaling

Firefox 150% Scaling

Chrome 100% Scaling

Firefox 100% Scaling