Thunderbird 38.5 is quite old. The current version is 52.5.2, so if you have just updated to 38.5 you must have, up till recently, been running an ancient version.

Why did you pick 38.5 in particular?

Anyhow, yes, there was a cosmetic change. Do you not find that the selected box changes to normal colours while you're typing into it? I suspect the intention was to make it obvious where the input focus is, perhaps in response to complaints that people sometimes found they were unexpectedly typing into the wrong box, or had experienced an unexpected response to use of the tab key.

One of the regular contributors on this forum (Toad-Hall) has, IIRC, presented some adjustments to revert to the previous colour scheme. I'll see if I can find that for you.

Hi Zenos, Thank you for responding. Thunderbird 38.5 is what keeps popping up asking me to download it. I did not want it but accidentally clicked to download it and I am not happy for several reasons. First is the cosmetics which I do not like.

Second and more of a problem is the font size. I have chosen to compose in Arial Large - but when I send the e-mail it gets converted to Arial X-Large. I tried all the settings to no avail. And it is not just on my computer. The same thing happened on my wife's computer. So, even though I was composing in a comfortable font size - my recipients were receiving X-Large font.

I uninstalled TB 38.5 and reinstalled an older version on both computers - and it is working fine again. So, I guess I will just stay with the older version which works fine for us. Thanks again for your response. God bless, Bill

Here is Toad-Hall's thread on this topic:

https://support.mozilla.org/en-US/questions/1115638

If you want to move to the current version, you can get it from here:

https://www.thunderbird.net/en-US/thunderbird/all/

The normal upgrade-in-place procedure doesn't always seem to take you on to a new version.

I'd quite forgotten about the coloured addressing and subject line boxes. Mine are white, but probably due to an add-on that colours the frame around these boxes as a way to cue me to think about which account I'm using. Here at work, business-related accounts get a green border, and personal accounts get various shades of red or pink to warn me I shouldn't be using them. I think this add-on restores the white to the text boxes too.

I would advise against composing using a large face. That is likely to lead to your correspondents getting messages in huge fonts. If that is what you and they want, then fine.

The better (IMHO) general solution is to set the display to your liking, and leave the font size in the delivered text at its default size, so it appears at whatever size your correspondents consider normal in their email clients.

So leave the size at "medium" in the composition settings, (Tools|Options|Composition), and in the display settings (Tools|Options|Display|Formatting→Advanced), set it as big as you want it. This setting applies to both incoming and outgoing messages, so you don't have to mess about with the zoom to see small text in messages sent to you. I'd like to point out in particular the "minimum size" setting.