Install Dorando keyconfig, then open the add-on's Options, Add new key, Name it Subscript, enter doStyleUICommand("cmd_subscript"); for the code, OK, then in the key box, key in something like Alt+Ctrl+L, Apply. Change subscript to superscript to make a superscript key. Same with strikethrough.

I downloaded Dorado but it's an.xpi file and i am using Windows10, so I don't know how to run it. Also the link to mozilla.dorando.at/readme.html appears not to find a page. I looked at http://forums.mozillazine.org/viewtopic.php?t=72994 but it doesn't help.

Drag the xpi file from File Explorer and drop it onto Tools/Add-ons/Extensions in TB.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thank you very much. After following the instructions carefully, it works

Now I tried to do the same for Superscript. I can get the name to appear in the Keyconfig pane but the key combination says <Disabled> and I can't change it. I've tried ALT+CTRL+U and ALT+CTRL+D but neither works. For 'code' ihave written: doStyleUICommand("cmd_superscript") but of course that might be wrong. Is there a way to find the correct code to use?

In my case, I have Alt+Ctrl+U working for Superscript. Make sure you have the command exactly as (see picture):

doStyleUICommand("cmd_superscript");

http://kb.mozillazine.org/Keyconfig_extension:_Thunderbird

Thank you for prompt response. I am using the code string as you advise. But the problem is that in the Keyconfig pane, the key combination is replaced by <Disabled> and I can see no way of changing that. If I 'Edit' the shortcut, on closing the editing pane, the Keyconfig pane has 'Superscript' replaced by the string 'key1518648626504', and still the <Disabled> will not go away.