After trying various methods, I successfully configured my version of Thunderbird to distinguish Cyrillic characters. Here is how I did it. First, let me show you the effect with an image. I pasted content from X and emailed it to myself for testing. After the improvement, it displays like this. Now it's easy to tell them apart.
Only the font for the Cyrillic 'a' has changed.
First, open the menu (three lines) in the top right corner of Thunderbird, then go to Help > Troubleshooting Information.
Click on Troubleshooting Information.
Next to the Profile Folder, click "Open Folder." The profile folder will open.
Press the "Open Folder" button next to the Profile Folder.
In the folder opened by Explorer, look for a chrome folder. If it doesn't exist, create one. Then, inside that chrome folder, look for userContent.css. If it doesn't exist, create it as a text file.
In my environment, I created these new files.
Open userContent.css with a text editor and add the following code:
1@font-face {2 font-family: "CyrMark";3 src: local("Courier New");4 unicode-range: U+0400-04FF;5}6* {7 font-family: "CyrMark", sans-serif !important;8}
Save the file, completely quit Thunderbird, and restart it. The email body display, which previously made it hard to distinguish characters, will now change as shown below.
The Cyrillic 'a' is now easily identifiable.
It might be a small comfort, but this should provide some peace of mind. I hope this helps someone.
