If you only ever use courier, then you may have an easier time of it if you set Thunderbird to compose using plain text. You can set it to display using courier, though your correspondents would see the message in their preferred or default font. All in all, I always think that how the reader sees my message to them is their concern, not mine, and I shouldn't expect any font settings that I use to countermand their preferences. If you approach it this way, then there's no point actually stipulating any particular font.

A lot of people have this font instability problem, yet I rarely see it myself, so I find it hard to make any useful comment on how to correct it. I believe that the Stationery add-on helps by inserting definitive font settings. Doing a few returns to create blank lines to buffer you from the invisible font setting closing tag can help too, but you must back up the cursor to the top of the message before typing.

Thanks for your reply, but the point of using courier is to align the text so that it is read properly by the recipient, I'm not so concerned about how it looks at my end.

But off my soapbox, to me it is a bug in the code. If one selects a font, one should be able to stay using that font until one switches to another font, and the program should not be allowed to modify it at will.

See attached image.

Yes, it is a bug. It's a long-standing bug, and no-one at Mozilla seems inclined (or able?) to address it. I've seen blog comments suggesting that the HTML editor in Thunderbird is considered to be unmaintainable. There have been suggestions that what Thunderbird needs is an option to plug in an editor chosen by the user, but so far, what I have seen of this offers a source code editor swap-in, rather than the wysiwyg editor that you'd probably prefer.

So, in an ideal world, we'd report the bug, it would be fixed and we're all happy. But that's not likely to happen and we are where we are.

My suggestion of entering blank lines before you start typing would probably help. But I'll come back to that.

Looking at your screen shot, one workaround would be to place the cursor between the y and the period in display. or select the period itself, and continue typing from there. Yes, you have to re-type the period, but you guarantee that the typing is taking place inside the zone where the Courier font is active. But I think you knew that already. ;-)

Another is to get yourself an HTML editor option (nic-nac has one, and the previously mentioned Stationery add-on has one too) and then you can switch to a "source code" view where you can place the cursor exactly where it needs to be. Inevitably, with a complex document using font changes, lists, tables and so on, this capability becomes essential.

Why do I say that? Because this problem is a bit deeper than it first appears, The white space that you type into can support both visible text and invisible formatting tags. If you move the cursor to the end of the text, how is the program to know whether you want the insertion point to be inside or outside the last-specified formatting zone? And whatever decision it makes on your behalf, ideally there needs to be a way to indicate to it that you wanted the other, alternative, option. Say you had set up a bullet-point list, went off to your sig then came back to the message body. In this case, you might want the next text you add to appear as normal body text, not as a new list item. (Though in practice you may want to be be able to make this choice yourself.) So here you'd want to be outside the closing list tag, whereas with the font you'd want to be inside the closing font tag. How is the editor to know which to do? Imagine you had a stack of closing list, font and style tags. You know where you want the insertion point, but how to convey this to the program? This is a fundamental problem with any text editor that uses inline mark-up.

In my experience, even Microsoft haven't got this right. If I open a new Word document, and paste in a table or similar from the clipboard, I find I have a document where I can't do much other than add to or modify the table; there is nowhere I can place the cursor that allows me to enter regular document text. It's the same problem of how to specify whether you are inside or outside the formatted area. Microsoft don't even offer a "source code" view (how I miss this feature that was in the old DOS version of WordPerfect!) and the option to see "invisible" characters such as paragraph breaks doesn't seem to help either.

In Word, I use the same workaround as I suggested for Thunderbird; first of all I enter three bank lines, then do my paste into the middle one. I now have blank lines before and after the pasted table where I can shoehorn in regular text. (Note; this may have got better in recent versions of Word, but old habits die hard; I always do the blank lines thing now, anyway. ;-) ) It's a pragmatic approach; give yourself some white space in which you're guaranteed to be able to hit the target.

There's a further bug in Thunderbird that might affect you. If you send an email message using the default formatting set-up, Thunderbird "forgets" to explicitly specify your chosen font. It will include stuff about size, but omits type face and family. (The use of the Stationery add-on fixes this.) So it's not uncommon for an Outlook user to see your sans-serif composition in Outlook's default Times New Roman. I was horrified when I first saw this, as I have a strong aversion to serif fonts on computer screens. However, as you appear to be actively selecting Courier, you should have some specific font-setting attributes in your messages, so hopefully this issue doesn't affect your messages.

Again, an HTML source editor would let you check on this for certain.

Zenos, thanks for the extensive and thorough response, I appreciate it.

It turns out that (unconsciously?) I have been doing most of what you suggest. No matter what, I will not succumb to switching to Outlook ;-)

I wish calendar would work better ...  :-(

I just spent nearly an hour going through all the suggestions how to fix that problem when it occurred to me shouldn't Thunderbird provide a solution and fix it instead of the users trying to jump through hoops to remedy their problem?

   I really think they should so we could type an e mail without going back and fort. I never experienced this in any other e mail programs.

Well, I'd point out that what is probably the most popular word processor in the world, MS Word, has similar troubles. These issues are inherent in the use of inline mark-up code.

Thunderbird has a novel solution to part of this problem; when editing an html mail for forwarding, then any tables used in its composition are outlined in red. And this is one feature that is frequently complained about, because most users simply don't understand what's going on. They want it to be simple, and there's no easy way to simplify nested attributes in text formatting. If you want "simple", use plain text. Zero formatting.

Maybe the wiki-like markup used here in this forum isn't so bad, because it uses visible tags. But then it's not WYSIWYG. We're trying to square a circle here.