Runbox 7 and Firefox on retina display

I’ve got a retina iMac, and the text in the message list pane doesn’t look crisp when running Firefox. The folders list looks fine, and so does the text in the preview pane, as shown in the attached screenshot. However, if I try to load Runbox 7 in Safari or Vivaldi, everything looks very good–crisp text. Is there some Firefox-specific bug on the Mac?

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I too notice this issue. On a PC running Firefox. It is not as pronounced as your screen shot, but i too notice that the writing has a fuzzy look to it.

In Chrome it looks great. Clear and crisp.