I'm not seeing this here on TB 60.4/W10. Does it happen when you cut or copy existing text in a message or text in another program? If it's with existing text, was that pasted from another program, e.g. Word, not typed into TB directly?

It happens in both cases: When editing an existing email, AND when pasting in text from Word.

Pasting text directly from Word is a possible source of the problem, as it introduces a lot of incompatible, hidden formatting. Try Paste Without Formatting (Ctrl+Shift+V) and with text that is typed directly into a Write window. Do you have any add-ons that might affect message composition?

I very rarely paste from Word.

The real problem is when I need to edit an email I am writing directly into Thunderbird. I place the cursor, click Paste, and the text shows up on the next line, which also obliterates an spacing between what I'm writing and the paragraph below.

All I know is that when Thunderbird updated itself this last time, I started having trouble editing emails in Write mode.

Try it in safe mode (hold Shift when you launch TB) to test the possible effect of add-ons.

I started Thunderbird this morning and went through the routine of starting it in Safe Mode without add-ons. I get the same editing problem--when I cut and paste from within an email, the text pastes on the line below where I place the cursor. If the paste is at the end of a paragraph, the double space is obliterated.

Thank you for helping me. I am not cyber-savvy.

I wonder if this is a word-wrap issue. When you enter text in a new message window, does the text extend to the edge of the window, or is it confined to a portion of the window, such as a section 72 characters wide?

It can be just a single word.