It appears that since Helvetica is not installed by default on Windows 10, that Arial is being used. And Arial (when slightly rotated) is not rendered very well.
If I change the rotation back to 0 deg, it appears less jaggy, but still not great:
Part of the story (in addition to Helvetica not being installed in Windows) is also the graphics acceleration defaults for Firefox, which you can find here:
If the machine does not have Acceleration on, then the issue is more likely to occur.