I have multiple email accounts in Tbird. I enabled the m.store.... switch in about:config and then changed server settings for each account from mbox to maildir. All email accounts appeared to successfully convert with the exception of the one large email account, it's nearly 3GB. TBird crashed after about 11% complete and closed. On restarting TBird, that large account showed mbox still for mail.store type. I changed server settings to maildir again and conversion stayed at 0 % for nearly half an hour before TBird crashed and burned again... On the third try, conversion of that email account succeeded, but TBird never restarted after completion. Within a few minutes of completing the last account, TBird crashed and the crash reporter popped up... That was several days ago... TBird crashes every few minutes ever since the mbox to maildir conversion of this large mailbox. I have uninstalled and reinstalled fresh TBird several times. I've started in Safe Mode... I've disabled or deleted all addin's,extensions, whatever. I've shut down AVAST antivirus, MS Firewalls, everything not absolutely necessary. TBird continues to crash every few minutes when started and running. While TBird is up and running, it works fine, gets all my emails downloaded, I can compose and send emails no problem. Hardware is stable for 3 years, no memory issues and nothing else crashes, no other applications, Windows 10 build 1809... That Win10 update was applied several weeks ago and I had no problems as a result of it... The only thing I believe I've changed is the mbox to maildir mail.store conversion. All the crashes reported are keyed to my [email protected] email account. Coincidentally, this is the large account that took 3 passes to convert. I check with my email server provider and they report no issues or problems with my account from their end... Any thoughts or feelings about what's going on? Thanks! Blaine Miller ... ... Phoenix, AZ (MST TimeZone) edited out email and phone# at end of post from public and search/spam bots view as this is a public forum and not a private ticket system or such.