The chrome.css solution doesn't even work anymore due to changes in Thunderbird!

See https://support.mozilla.org/bm/questions/1269283 for an answer that works: changing the pixel scale factor from 1.00 to 1.50.

Thunderbird should be fixed to work like any other app. Other apps don't show up with tiny text on large resolution screens!

Also, there are lots of posts recommending installing an addon that also doesn't work anymore! Bugs should not be fixed by addons.

There should be central documentation that is always up to date, and bugs should be fixed promptly. There should be standard central build procedures.

I would have been fired for releasing a product with a regression bug, particularly one like this that has been reported again and again for years.

userChrome.css is not working either.

I must say that I did not experience any issue with font size after update, but many people seem to have had some font size issues.

Just in case anyone wants the full info: The following option was added back into Thunderbird to assist everyone who needed control over general UI font. Changing this will effect all the general UI font including email content for received and composing.

• 'Menu App icon (3 lines) > Options > Options > Advanced > 'General' tab
• click on 'Config Editor' button

It may say 'here be dragons' as a type friendly warning to be careful :)

• In top search type: pixels
• Look for this line: layout.css.devPixelsPerPx

The default is -1.0

• Double click on that line to open a small window.
• clear contents and type in eg: 1.2 or 1.25 or 1.5
• click on OK

I'm using 68.3.1 and I use 'userChrome.css' in 'chrome' folder. But it depends upon what you are trying to increase in font size. Global UI font effects Folder Pane, Thread Pane, Headers, Menus, Calendar, Account Settings, Address Books, Write window. Does not effect: email content in received nor composing and due to recent changes: content in tabs - does not effect 'Options', 'Addons', About Profiles etc.

/* 
* Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

/* set default namespace to XUL */

/* Global UI font effects Folder Pane, Thread Pane,  Headers, Menus, Calendar, Account Settings, Address Books, Write window. */

/* 18px can be changed to personal preference*/

* { font-size: 18px !important;
} 

Thunderbird no longer uses xul, so you'll need to comment out the namespace line.

If you have trouble with text that's too big as well as too small, then PixelsPerPx does little good.