Does it still happen with hardware acceleration disabled?

Made no difference. Neither did disabling WebGL.

Go to about:config and try changing gfx.e10s.font-list.shared and gfx.font-list-omt.enabled to false then restart the browser.

Do you have any Hanazono fonts installed?

Issues with a weird looking font or missing glyphs can be caused by a problem with a specific font and you need to identify this font and fix this. You may find that it can be resolved by finding and removing duplicate fonts installed in the local ~/Library/Fonts folder, that are conflicting with standard system fonts.

You can check in Font Book for font issues like corrupted and duplicate fonts. Note that you should be careful about disabling "Allow pages to choose their own fonts, instead of your selections above" as this will cause issues with iconic fonts used by webpages to display small icons (you may see text labels instead of icons).

@cor-el

This doesn't make sense to me. If it were a problem with duplicate fonts. Wouldn't this be a system wide rather than Firefox specific problem? It's not a bad idea, but, it doesn't seem like a Firefox specific answer either.

I've got Opera and Chrome browsers installed as well and it's not a problem with either of them.

@zeroknight:

What's a Hanazono font? Is it an, if I did I would know font? Or is there something specific I should / could be looking for?

Remove any duplicate, old, outdated and unused fonts.

Bug 1803406 details the issue:

Core Text will resolve font names differently in the content vs parent processes when duplicate fonts are installed (e.g. old versions of Arial, Helvetica, etc that some users have "inherited" from ancient systems). The mismatched fonts used for layout (in the content process) vs painting (by the parent) lead to the "garbled text" issue.