Does it happen in safe mode? Hold Shift when you launch TB, or Help/Restart with Add-ons Disabled. Quote and Compose Manager is one add-on that can cause this.

Thanks - that was the problem. I've now reverted to 52.9.1 as I found there were 6 useful add-ons that were incompatible with v.60, and everything works fine again.

I can understand reverting to 52, but many add-ons that are labelled 'incompatible' can in fact be made to work in 60 with a simple edit, and others have been rewritten by volunteers to be compatible. If you wish to list your add-ons here, we may be able to propose workable solutions for TB 60.

Sure, here are the add-ons that TB 60 listed as incompatible. Except for QuoteAndComposeManager I haven't checked to be sure they're truly incompatible.

Any Confirmation Message before Send Change quote and reply format ConfirmFolderMove Identity Chooser QuoteAndComposeManager XNote++

Any Confirmation Message before Send doesn't work in TB 60, but some other options are Check and Send or Correct Identity. Identity Chooser 1.9.7 should be compatible with TB 60.

Alternatives to Change quote and reply format: ReplyWithHeader and TotalMessage.

ConfirmFolderMove 1.2.0 should be compatible with TB 60.

QuoteAndComposeManager breaks Reply and Forward in TB 60, but some functions available with ReplyWithHeader.

XNote++ 2.2.14 should be compatible with TB 60.

Thanks for the info. I'll check out the alternatives you suggested.