Same for me, even I set "layers.acceleration.force-enabled" to true. Chrome and Chromium are smooth.
EDIT: Turning "High Resolution" (in the settings of the webpage on top right) off and on seems to fix the issue somehow? It's smooth now, but reloading takes it back to choppy.
The slowness in firefox comes from the station name drawing.
Going from U to S+U and back again seems to reset the setting for drawing them.
The settings option for drawing the station names is somewhat buggy.
To optimize this the text should probably be drawn to another canvas/surface and layered behind the main one and not be updated that often.
It's also smooth for me (also Chrome) in "U-Bahn only" mode, but when I switch to "U+S", it's noticeably less smooth (10 fps?). It gets better if you deactivate the "city map background" in the settings.
EDIT: Turning "High Resolution" (in the settings of the webpage on top right) off and on seems to fix the issue somehow? It's smooth now, but reloading takes it back to choppy.