I also found the snap had trouble rendering fonts correctly. Below you can see, on the left, Firefox 60.2.0esr running as a native package in Debian stretch and on the right, Firefox 62.0 from the snap. The versions are different but the problem was ocurring before the 62 snap upgrade.
We see better the effects when the image is viewed in full screen.
Also, this seems to be a bug in Firefox itself that makes it fail to use the proper fallback font when certain metrics are used.
Zooming the page in and out can trigger the bug, for example, although it’s not clear to me why this would happen in the snap and not outside.