The latest update to my Firefox browser produced an unusable interface and content area: all instances of where a character should appear were dark garbled blobs of pixels. In menus, in the viewing area, totally unusable:

enter image description here

So I first searched for obvious bugs reported to Mozilla. Found nothing.

Restarted with a new profile. Everything fine again. So the bug is local configuration. But a completely fresh profile is not a desirable option.

After a few iterations of altering contents of the old profile folder I found that neither addons nor cache files were the culprit.

A fresh prefs.js file is the one file that after re-creation lets me use my entire old profile, including addons.

But it also holds all of my customisations (of which one will be responsible for the rendering catastrophe). The new file is now 21.989 bytes vs 77.135 bytes fro my customised prefs.js.

What I thought I could do is going through the old one line by line and re-create all settings I want until it breaks again. That's likely taking quite a while.

Another option would be to delete everything I identified with having to do with rendering, which I did, but obviously I must have missed one, as that didn't work out.

Deleting one half of the prefs and re-iterating this bisection seems to break the browser prefs file.

How do I isolate the offending line (or lines) in prefs.js causing the problem most efficiently?

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