User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36

Steps to reproduce:

Thunderbird 102.9 and Windows10.
Select any received eMail, content is displayed in message preview window
Choose View/ZoomIn, message content size is increased, it doesn't matter if
ZoomTextOnly is selected or not.
Choose View/FontSize/Increse font size, message font size is incresed
Close thunderbird
Start it again, select same message
Font is back to default one (too small)

Same applies to new message window or viewing existing message (new window (double click on message), message is shown in new window, increasing or zooming font size increase font only for that session. After restart it's back to default/small size which is unreadable on large monitors.

Actual results:

After restart, all font size changes are lost, message content in preview pane is back to small one.

Expected results:

Zooming in or increasing font size should remember set level after thunderbird is restarted.

Are you sure version 102 is not affected?

See Bug 1689030 - Preserve zoom option

I forgot Bug 1654069 - Zoom level of add-ons tab does not stay after restart

But, this bug report is only about compose?

Thunderbird 102.9.1; 64bit version, it's definitely not remembering zoomIn or increase in font size after restart.

(In reply to Wayne Mery (:wsmwk) from comment #1)

Are you sure version 102 is not affected?
See Bug 1689030 - Preserve zoom option

Not a Supernova regression. Daily 113.0a1 (2023-04-05) (64-bit) has exactly same behaviour as TB 102.9.1 (64-bit) on Win10.

Actual results

Font size

• App font size is generally preserved across TB restart, both 3-pane and Write window, for the UI; message body is different for both.
• 3-pane message body: Increased app font size works, but is not preserved across TB restart (that's a bug, this bug).
• Composition's message body: Font size never increases with the UI. Then of course, there's no different font size to restore, because it never changed in the first place.

Zoom

• Message pane body zoom works, but is not preserved across TB restart
• Composition body zoom works, but is is not preserved across TB restart

As Wayne said, Zoom is Bug 1689030 and Bug 1654069 territory, maybe better to keep those issues separate (out of this bug).

Expected results

• Changed font size for message reader's message body should be preserved - I guess that's the whole point of increasing your font app-wide.
• Similarly, perhaps the font size of composition's editor should also be increased with the app font size - dito, users increase fonts for better readability, so they'd certainly want that to last across sessions. I'm not 100% sure, see my last caveat below.

Caveats:

• It's worth noting that if you have 3 message tabs open, each of them can be zoomed independently, which is good, and we don't want to break that.
• Not sure then if the message preview pane should preserve its zoom factor, because again that might be deliberately different or even temporary for each message.
• Perhaps composition's body font is not increased by design? Having a font size different from what you'll be sending might be somewhat confusing, but yeah, otoh, if you need a bigger font to be able to read what you're writing, there's little choice. Both Zoom and Font size can be reset at any time to check out the actual font size (as it displays on your screen).

This is a problem for a lot of users. In firefox we have have a setting for default zoom:

https://www.howtogeek.com/657255/how-to-set-a-default-zoom-level-in-firefox/