No idea, I have no issue typing that into a message body. I just tried it.

Thank you, Matt. Do you have any idea why I am having this problem? Does anyone?

The appearance of my e-mail toolbox has changed too.

Thank you for any ideas you may have.

Run in safe mode (hold Option when launching TB). Can you now compose in the Write window?

I tried your suggestion, sfhowes, and it did not work.

What I find is that I can type XYZ in the uppercase but cannot in the lowercase xyz, What happens is that x is written " and yz are nothing at all.

Any other suggest would be much appreciated.

Under Options(Preferences)/General, is hardware acceleration enabled or disabled (at the bottom) in normal mode?

sfhowes, you ask: if "Under Options(Preferences)/General, is hardware acceleration enabled or disabled (at the bottom) in normal mode?"

In Preferences at the bottom of the page, under "Indexing, "use hardware acceleration when available" is checked, enabled. I don't know about "normal mode" -- the term does not appear on the page. Should "use hardware acceleration when available" be checked (enabled)?

I am technologically challenged so please bear with me. Thank you.

Normal mode is when you start TB as usual, not in safe mode. Since safe mode disables hardware acceleration, and you have it enabled in normal mode, it appears to not be a factor with your issue.

In case the problem is due to a graphics driver, I suggest you start the mac in safe mode, and then run TB, as OS X safe mode applies basic drivers.

https://support.apple.com/en-ca/guide/mac-help/mh21245/mac

dear sfhowes — I followed your instructions and while on "safe mode" I could type xyz in an e-mail (though New York font which I normally use showed "not installed")

Then I restarted my Mac to leave "safe mode" and tried to write an email on Thunderbird but again xyz would not type.

I have an old Mac: OS X El Capitan version 10.11.6 if that is any help.

Your help is very much appreciated.

Look at the programs that launch at startup, as these are bypassed in OS X safe mode. Security, antivirus and online backup apps are typically sources of issues. If none of them are causing the problem, see if there are driver updates for your graphics and network adapters, especially the former, as safe mode applies a basic set of adapters, different from the ones used in normal mode.

Most of this covered here: https://support.apple.com/en-us/HT201262

Dear sfhowes — I tried your recommendation and it seems none of these are the problem. You say to "see if there are driver updates for your graphics and network adapters, especially the former."

I do not know what driver updates are or where to find them. Please advise. I do use Adobe Photoshop CS5 quite often.

Apparently, the graphics driver is only updated when OS X is updated. What is your version of TB?

My version of Thunderbird is: Thunderbird 78.4.0 (64-bit)

I don't know if you can update your OS X, but that would be preferable to reverting to an older version of TB which may not have this problem. I haven't seen reports of other mac users, using either TB 78 or El Capitan, facing this issue.

Thank you. Might this be a hacking problem? Would you have any recommendations or advice? I would be most thankful.

Can't say if it's a malware issue. You could run anti-malware utilities, which I'm not familiar with on OS X, or maybe create a new profile, add an account, and see if the problem persists. Help/Troubleshooting, about:profiles, to create and manage profiles.

I would suggest you try another font.

I followed your instructions and while on "safe mode" I could type xyz in an e-mail (though New York font which I normally use showed "not installed")

As you are using HTML, if New York is not installed you will be using one of the fallback fonts. It is possible that the font being used as a fallback simply does not have those text characters in it.

Matt — thank you. Your suggestion seems to work. Please help me with these questions:

I went to Thunderbird preferences and clicked on "restore defaults" and what came up for font was "variable width" and when I went to fonts, I found also "fixed width." What do these mean?

Under that, I find: Helvetica, Ariel; Times; and Courier at the top of the long list of fonts that follow. Are Helvetica, Ariel, Times, or Courier the recommended fonts?

Thank you kindly for your help.

I don't know if you can update your OS X, but that would be preferable to reverting to an older version of TB which may not have this problem. I haven't seen reports of other mac users, using either TB 78 or El Capitan, facing this issue.

I'm also a Mac user and I have this problem too; I posted about it a few days ago but didn't receive any responses.

I recall there was a TB user not too long ago who fixed a problem on a mac by resetting the OS X font cache. It's as simple as starting in OS X safe mode, and can't do any harm.

https://discussions.apple.com/thread/251381930

Matt — thank you. Your suggestion seems to work. Please help me with these questions: I went to Thunderbird preferences and clicked on "restore defaults" and what came up for font was "variable width" and when I went to fonts, I found also "fixed width." What do these mean? Under that, I find: Helvetica, Ariel; Times; and Courier at the top of the long list of fonts that follow. Are Helvetica, Ariel, Times, or Courier the recommended fonts? Thank you kindly for your help.

Fonts is a truly complex topic. one I am not an expert in, but I will try.

Fixed width fonts are that where each and every letter in the font occupies exactly that same amount of space horizontally and vertically. If you are old enough typewriters used fixed width fonts. See the two examples below.

Thus is in a fixed with font

This is variable width

Emails fall into two categories; Plain text and HTML.

Plain text is just what it appears and can best be described as what you can write using the windows notepad. Except that it will always use a fixed width font

HTML is the type of email most people expect. It has fonts and font names and you can make the text larger and smaller or bolder. It vaguely resembles a word processor, but is limited to the output being destined for a web browser. Just accept they are very different, despite some obvious similarities. It really does help with expectations.

A well formed email will actually have both parts, plain text and HTML, as there is no guarantee that the person reading will have access to a full on web experience with email. You can see this in action on the Menu. View > message body as and change around between the default HTML and the plain text.

That is a lot to get to your question about fonts. But is is really essential background. HTML uses font families because again, just because you have Arial. Tthe phone reading your mail may not. All fonts fall into a font family ( A bit like naming of animals) So all fonts fall into one of three highest level families. (Serif, sans serif and monospace). Now we get to the lists of fonts and their being recommended. The defaults Thunderbird uses are old. Decades old really with the occasional touch up. But they represent a set of "safe" fonts that you can fairly realistically expect most readers to have on their computer or phone. Or a very close substitute defined in the font family. They also represent fairly common fonts use to send mail to you for the same sorts of reasons.

Now to the specific New York font. This is an apple only font, so no one you ever sent and email to probably saw it as you did. They would have seen it as one of the substitute serif fonts (as it is a serif font) on windows that would usually be "Times New Roman" or for those using apple devices "times" as the original New York is a bit mapped typeface it is not really suitable suitable for email. But more to the point Apple re-released it in 2019 for the sole purpose of developers mocking up APPLE user interfaces. So you probably no longer actually have it installed for use in email at all. See https://en.wikipedia.org/wiki/New_York_(2019_typeface)

But it is available for download here https://developer.apple.com/fonts/