I'd suggest you open the config editor and set the css.layout.dpi setting to a value suitable for your display. Its default will be -1. Start by changing it to 96, then experiment to find which way to change it to suit you. It probably needs to go upwards, by 1 at a time.

This will change the size of all text on Thunderbird, that is, in menus, in lists and in message text.

Zenos said

I'd suggest you open the config editor and ...

I know nothing like about:config in this client. Does it appear in a menu somewhere?

p.s. both this and another even more substantial answer provide information for Mac OS. Odd that the second assumption is that we are not using PC/Windows.

Tools|Options|Advanced|General→Config Editor.

That's for Windows. We can see some info about the system used by the original poster of a thread. When another poster jumps in to join in, as you have done here, we can't see what YOU have. Replies for Macs are usually done in response to squeals about menus not being as described.

http://kb.mozillazine.org/Menu_differences_in_Windows,_Linux,_and_Mac

I would urge anyone using software and wanting to be proficient with it to learn its menus. Then I'd hope I could write "go to account settings" without having to explain the myriad number of ways of opening them.

In this case, it's "go to options" and just to confuse matters, linux and mac folk need to look for "preferences". Bah!

The op in this thread was using:

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:58.0) Gecko/20100101 Firefox/58.0

Zenos said

Tools|Options|Advanced|General?Config Editor. That's for Windows. [Extensive additional information concerning the differences in menus for 3 different OS.] I would urge anyone using software and wanting to be proficient with it to learn its menus.

"Learn its menus"? In fact what you mean is "You should know every single item in every single tab of every single section. The way I do. Because I spend many hours each week with this. And I know 1000 times what you do. So you better eat that and choke." I think I've read you correctly here.

Then I'd hope I could write "go to account settings" without having to explain the myriad number of ways of opening them.

This wasn't "Go to Options" ... you exaggerate in order to mock. You and Trump should be buddies; you share that attitude.

T)ools; O)ptions; Advanced; General; config editor lower left. Set css.layout.dpi That. That's all it would take.

1. T)ools
2. O)ptions
3. Advanced
4. General
5. config editor lower left. Set css.layout.dpi

But you need to pontificate, because you're yet.another Moz.******.******** ... and you want everybody to know it. Shameless.

In this case, it's "go to options" and just to confuse matters, linux and mac folk need to look for "preferences". Bah!

Bahhhh indeed ...

p.s. people who are sane enough to take offense at this contemptuous arrogance don't stick around to talk about it. "'Tell a fool he's being foolish and he'll call you a fool'." I must be simple minded: when I hit a snag I come here ... forgetting just why I've avoided you and this place for years ... (After Netscape "released the code" we had a good couple of months. Because there was actual heavy lifting to be done, and lots of it, twits and twats were nowhere to be seen. A good era.)

1. open the config editor
   1. T)ools
   2. O)ptions
   3. Advanced
   4. General
   5. Config Eeditor is lower left.
2. set css.layout.dpi setting to a suitable value
   1. Default is -1. 96 should be a good start

NB1: there is not css.layout.dpi in Thunderbird. There is 1 single "dpi" entry. The single dpi entry: layout.css.dpi

NB2: Changing this to 96 had no effect at all. (Changed the integer; closed the editor; closed Options; exited TBird; ReStarted; #NoJoy) Other values had likewise no effect.