I guess you're right. I see the same on my desktop Firefox on Linux. But on my Android phone it's pretty smooth.

It seems to just not work in Firefox for me, it's the same for me on FF Android but in Chrome it's beautiful! Constant text size while zooming and it's super smooth.

Edit: It also weirdly doesn't work on Chrome on Linux for me either, only Chrome on android. Oh well, hopefully they can bring support everywhere eventually.