I've never seen that happen, in either firefox or thunderbird. So I tested it in thunderbird 149.0.2 (64-bit) as the second word of a sentence. It remained properly lower case. I'm also posting especially from firefox (also 149.0.2) and noticing it is not happening there either.

Do you have any addons, user dictionaries, spell checkers etc? Try running the troubleshooter mode (main menu > help > troubleshooting mode) which will restart without addons to confirm whether one of them is the cause.

My only add-on is an extension to schedule email messages. I tried Troubleshoot Mode but the same thing was still happening. But somewhere there must be a setting to automatically capitalize that E.

It doesn't happen in Word or Pages. It seems to be exclusively Mozilla related.

Do you have a dictionary configured at Menu > Settings > Composition? I'm not sure if troubleshooting mode disables dictionaries.

There is a dictionary "English (United States)" checked, but if I uncheck the box, the "Esp" thing still occurs.

Found it! It was under the Mac System Settings > Keyboard > Text Input > Text Replacements...

Not sure why that didn't affect Word or Pages. Anyway, it's no longer occurring in Thunderbird.