I just began having this problem. I am running macOS 10.12.6 and Firefox 57.0.1.

One site where it occurs is daringfireball.net (see screenshot). That website displays without problems in Google Chrome.

You'll see that there are two sections where text isn't displaying properly. For the first section (h1), where the type is larger, the rule is font-family: "Gill Sans MT", "Gill Sans", "Gill Sans Std", Georgia, serif; and .LastResort is being substituted. For the second section (h2), the rule is font-family: "Gill Sans MT", "Gill Sans", "Gill Sans Std", Verdana, "Bitstream Vera Sans", sans-serif; and, again, .LastResort is being substituted. When I toggle off those rules, the text appears correctly (in Verdana, which is the font specified in a higher-level (body) rule).

Georgia, Verdana, and Gill Sans MT are all installed. I have tested them and they are not broken.

I tried changing security.sandbox.content.level from 3 to 1 and that had no effect.

From this thread I got the suggestion to try adding a rule. I added font-family: Verdana !important; and the text switched from .LastResort to Verdana and displayed correctly.

I've also consulted this thread and this thread which seem to be about the identical problem but found nothing that made a difference in them.

Something must have changed or gotten broken, because I previously had no such problem with the same browser and OS (unless, for some reason, I just never went to a site that displayed the problem before).

One final piece of conceivably relevant info: I manage fonts on my computer with Suitcase Fusion, which seems to be true (but not all) of the other users who are having this problem.

Any suggestions?

I believe my issue was a font problem. I use Font Agent Pro. Although, I was able to use Google Chrome with no issues what so ever (Firefox and Safari had this same font problem). I finally ended up taking font book back to "Restore Standard Fonts." I uninstalled/reinstalled Font Agent Pro. So far this has fixed this problem, but I haven't been using it this way for very long. Fingers are crossed this fixed the problem for good.

@dhnyny, @mrquig: Font Agent Pro issues from 57.0.1 were fixed in Firefox 57.0.2 in bug 1417420. Could you please update and let us know if your problem is now resolved?

Thank you!

I just updated to 57.0.2. The problem is not fixed. I'm using Suitcase Fusion 6, not Font Agent Pro, so perhaps the bug 1417420 fix doesn't address Suitcase Fusion users?

I reported earlier today that 57.0.2 didn't seem to fix the problem I was having. But I just rebooted and now it seems the problem is gone. Yippee.