For some reason the image didn't attach, here it is.

Anyone?

Firefox would normally retrieve this list via internet as set via devtools.devices.url.

• devtools.devices.url: code.cdn.mozilla.net/devices/devices.json

So make sure that you aren't blocking this request.

This could also be a problem with IndexedDB.

Current Firefox releases with multi-process enabled store device data (Add Device) in an IndexedDB SQLite database file in the object_data table as BLOB data.

You can check the storage folder in the profile folder

• storage -> permanent
  indexeddb+++fx-devtools/idb/478967115deegvatroootlss--cans.sqlite

Thanks very much for this! I don't think I'm blocking that request as I can load it fine in the browser. I think the issue might be the sqlite DB, as if I attempt to connect to it with sqlite3 in Python I get:

SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

But perhaps I am misunderstanding and trying to do the wrong thing, the fact that there's an issue with the first few bytes suggests the format is something different entirely. I'm not very familiar with sqlite.

Is this DB something I can delete so it can be generated again, or do you have other ideas?

Aha! I've been upgraded to Firefox 55.0.3 now and the list has been restored. Who knows what happened there, but I suspect corruption from installing Nightly.