If using Windows, you can enter 'character' in search box on taskbar and a character map pops up and you can select a character, copy and pasted into Thunderbird. There is also an addon for some predefined ascii characters - unicodify, and the other option that I'm aware of is to use the keyboard by carefully defining the character, but that may vary by OS. Also, if using HTML messages,, there is Insert>Characters&Symbols from the menu bar. And there may be other options.

For W10/11, try the emoji/symbol menu - Windowskey+. (period).