@whm I tried to replicate the issues you are having with both Firefox 67.0 (the latest production version) and 68.0 (developer version), but there are no visible issues that makes it particularly slow.
One culprit for the slowness issues might be plugins. Can you try disabling all plugins and see if you still experience the “slow script” prompt?
Regarding the “transparent” font, this might be due to some browser settings you have, or maybe the font is being blocked from loading and falling back to the natively present fonts. We will very soon release an update to the dashboard that changes the fonts used everywhere, and although unlikely, it might fix the issue.
Maybe you can try with a new installation separate from your default one, without any plugins or settings, and see if that fixes your problem.