See image below as guide.

• Click on the 'Menu app icon ' (3 horizontal lines') and select 'Options'
• 'Options' opens in a new tab
• On left side select 'Composition'
• Scroll down to 'HTML Style' section
• uncheck the checkbox so not selected 'Use Paragraph format instead of Body Text by default'

Restart Thunderbird to ensure all settings are properly saved.

Line 3 says: On left side select 'Composition' . I'm sorry. On my version of T-B there is no 'Composition' on the left side. All that's available on the left side are the following:

    General
    Home
    Search
    Privacy & Security
    Sync

    Extensions & Themes
    Firefox Support

Now where do I go?

Line 3 says: On left side select 'Composition' . I'm sorry. On my version of T-B there is no 'Composition' on the left side. All that's available on the left side are the following: General Home Search Privacy & Security Sync Extensions & Themes Firefox Support Now where do I go?

You are only seeing those options because you are looking at the 'Mozilla Firefox' browser Options.

The issue regarding composing emails is in 'Thunderbird' email client and not the Firefox browser. So you need to follow the instructions and apply them in Thunderbird.