Does it look correct in safe (troubleshooting) mode? Hold Shift when launching TB. Is hardware acceleration (Preferences/General/Indexing) enabled in normal mode?

Yes, it looks okay in safe mode.

If hardware acceleration is enabled in normal mode (it's disabled in safe mode), turn it off, restart in normal mode, and see if it still looks OK. If not, launch again in safe mode and 'Reset toolbars & controls'.

Turning that off fixed the display problem. Thanks! Any ideas why my signature isn't applied to emails generated by Quickbooks? In QB I click a button to email an invoice and it opens a new email in Thunderbird but the signature isn't there, like it is when I start a new email within TB. That used to work on my old computer also.

I don't have QB to test, but I would expect if a signature is defined for the account (each account has its own signature) that is called by QB when sending a message, the signature is applied. If you right-click a file in File Explorer, Send to - Mail recipient, does it launch a new message with the file attached, and is the signature included?

No, right-click send to mail recipient launches a new message with file attached, but no signature.

I just tried Send to - Mail recipient, and the file was attached to a message from the default account, and the signature was included. It also worked sending an image from my image viewer app. TB 91.1 32b/W10. My signature in this case is a simple html one entered directly in Account Settings, not an attached text or html file.

I presume you've checked that the default account has a signature defined. Do you have any add-ons related to signatures? As a workaround, you could use an add-on like Quicktext to add a preset signature manually.

I tried copying my HTML from the attached signature file and paste directly into account settings, but it still didn't work. I already have QuickText and will just use that as suggested.

Thanks for your help.