Thunderbird locks up

Hi, I am aware that this might not be an appropriate topic for this list, but you never know. I am running TB on Fedora 23. Have been using TB for years without problems. Suddenly when I launched TB the little wheel kept spinning and still does. I can't send or receive email. Nothing has changed in my account settings nor was there an update of TB. Ik can still handle email via webmail on this account. This is a message in the error-console: Timestamp: 05/31/2016 02:57:52 PM Warning: Unknown property 'grid-auto-columns'. Declaration dropped. Source File: resource://gre-resources/ua.css Line: 175, Column: 19 Source Code: grid-auto-columns: inherit; I don't know if this explains the locking up. I tried to remove and reinstall TB, but the problem remains. I asked this question at the Mozilla support forum, but this does not seem to be a useful forum. I don't know how to solve this problem. Does anyone have a clue? Thanks in advance, Peter