You can changes the font size for the entire Thunderbird interface, including the menu, folder list etc.

Click the Menu button in the upper-right corner of Thunderbird.

Look for Font Size in the menu.

Click the + button to increase the font size.

For more info see Thunderbird 115 Supernova FAQ

To increase the overall UI font size:

• Click on the Menu app icon (3 lines) > Font Size
• Click on the + icon to increase, repeat until folder pane and message list displays better for you.
• The number in the middle indicates the current pixel size eg: 14px

If you prefer to increase the size so it's a bit bigger for reading mail or composing or even apply some control to stop silly small fonts:

• Settings > General
• Scroll to 'Languages and Fonts'
• Click on 'Advanced' button

The 'Fonts' window opens.

• Fonts for: 'Latin'
• Set the Proportional Size: to say a size slightly larger than you chose for the UI Font Size eg: 16 which means 16px
• Set the 'Minimum font size' to eg: 12px (this must not be greater than the proportional size - it forces tiny fonts to use this as minimum)
• Uncheck 'Allow messages to use other fonts' - I advise this as you have more control.

• Now set up exactly the same for 'Fonts for: 'Other Writing Systems' (usually at bottom of drop down list) - do not omit this as it effects Unicode encoding.

So both 'Latin' and 'Other Writing Systems' are set up identical

• Then click on 'OK'

See example image below.

Please note: whatever size you selected for 'Proportional' eg: 16px that will now be the what Thunderbird calls your 'Medium' setting.

• Settings > Composition

Please use the default which should be:

• Font : Variable width Size: Medium

that means it composes using a prefered setting for you and does not impose it on people who receive your emails.