Couldn't load XRE functions.

Since the latest update, that's the error message I get about 15 seconds after the boot splash - FF GUI never even appears prior to the error box.

I tried reinstalling the PA platform as suggested in the previous unstable post. That worked with a fresh copy of Firefox, but not with my old copy. As I said, this problem presents BEFORE the GUI ever loads, NOT after the program is running. Looks like I'll be retrieving my February backup and postponing the upgrade until I find the culprit. What are XRE functions? I have several extensions and knowing which ones to target first might save gobs of time.

Thanks in advance...