You won't need any programming skills, there's a checkbox in the Options dialog:

"3-bar" menu button (or Tools menu) > Options

At the top click Composition, then the General mini-tab

At the bottom of the HTML section, uncheck the box next to "When using paragraph format, the enter key creates a new paragraph" and click OK to save the change.

Success?

Thanks a mil for the fast response and working answer to my dilemma.

Had tried that same Tools/Options tab and even the general small tab, too, but the language for the proper choice (which you supplied in clear English!) down by the HTML was foggy at best - at least to me, a non techie.

So followed your instructions, and, voila! I'm now back to my old comfortable TBird composition procedure and it works just the way it did before the upgrade to version 45. You've made me happy with TBird again.

Much appreciated ! :-)

I had the same problem. Your solution works just fine.

 My question is why was it like this in the first place? 

New paragraphs don't need double line spacing. They do need to be indented, which this option did not do.

Hi dtsaudio, in earlier versions of Thunderbird, the Enter key inserted a line break. Now (by default) it inserts a paragraph break and Shift+Enter inserts a line break. This was to conform with the way most HTML editors work now.

Indenting would be a question of the style rules for each paragraph. I'm not sure if Thunderbird has an easy way to do that (for example, if hitting the Tab key once at the beginning of the new paragraph will do it) or whether something trickier is required. Maybe start a new question on that one?

No need. As long as I have it back the way it was. Thanks for your help.

@jscher2000

Since I have a Dutch interface, I can't find the menu structure and options you recommend to use. Are screenshots possible? It would make it easier to understand. regards.

Kos

Hi Kostenzer, I saw this image in another reply:

https://support.cdn.mozilla.net/media/uploads/images/2016-05-03-09-51-16-401fcd.png

For the dutch interface, you first need to display the menu-bar. At composing a message, right-click on the composing-bar to tick the menu-bar. On the menu-bar go to Extra=> Opties => tab Opstellen => HTML => Un-Tick "Bij gebruik van tekstopmaak Alinea, creeert de Enter toets een nieuwe alinea".

By the way... When in the dubble spacing mode, extra spacing can be skipped by using Shift-Enter, when creating a new line.