When viewing the address book in horizontal density, the lines touch/overlap.
In vertical density they do the same, and are so hard to distinguish.

Adjusting density to Touch fixes the horizontal, and almost the vertical, but then the all the rest of the UI is expanded horribly for a desktop.

I'm running Linux Mint Cinnamon 20.3. I have a Windows 10 desktop that does not have the issue. I have the latest 102.3.2 installed.
Attached is a screenshot of the vertical view.

To fix it, I modified omni.ja.
/chrome/messenger/content/messenger/addressbook/aboutAddressBook.js
at line 1212
static densityChange() {
switch (UIDensity.prefValue) {
case UIDensity.MODE_COMPACT:
AbCardListrow.ROW_HEIGHT = 52; /* Vertical */
break;
case UIDensity.MODE_TOUCH:
AbCardListrow.ROW_HEIGHT = 74;
break;
default:
AbCardListrow.ROW_HEIGHT = 58;
break;
}
}

and at line 1312
static densityChange() {
switch (UIDensity.prefValue) {
case UIDensity.MODE_COMPACT:
AbTableCardListrow.ROW_HEIGHT = 30; /* horizontal */
break;
case UIDensity.MODE_TOUCH:
AbTableCardListrow.ROW_HEIGHT = 54;
break;
default:
AbTableCardListrow.ROW_HEIGHT = 42;
break;
}
}

This fixed the issue for viewing in compact density in my Linux machine.
Both vertical and horizontal views. Now these numbers might be off one or two for condensed, and I just arbitrarily adjusted the default and touch #s. I only care about condensed though.

Also bug 1780909 seems close but I stumbled onto this solution before seeing it and actually I'm not having the descenders issue.

User agent is Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.2
OS is Linux 5.4.0-126-generic #142-Ubuntu SMP Fri Aug 26 12:12:57 UTC 2022

It seem most important to have these details "default font in Linux to 16pt. I'm using a 1920x1080 screen and need the font larger in Linux to make all applications readable." from https://support.mozilla.org/en-US/questions/1391942

I do something similar on Mac and Windows, and I don't see this problem.

Francis, can you reproduce this?

From the developers ...

This is a known issue and known architectural flaw of the new treelistbox. Basically in order to recycle UI items and make the whole widget performant, all rows have to be the same height, so the easiest initial solution was to set fixed height values for the 3 density variations. This obviously doesn't work well if the font is increased as the widgets don't resize. We're tackling this in the new about3pane.

In short, this cannot be fixed in version 102. It will be resolved next year in version 115.

Have you tried setting the font size in Thunderbird using View > Font ?

After issue reported in Support Forum.
I did some tests. I'm adding info because this is not just a Linux problem. I can reproduce in Windows OS.
Windows 10
Version 102.5.1

Problem occurs in the Address Book list of contacts in 'Vertical View' only.
Note: You need to be viewing the Folder Pane and messages to get access to menu for font size. Oddly this is not available when focus is in the Address Book.

To reproduce issue:

• Set : View > Density > Compact
• Increased the font size: View > Font Size > Increase font size
  I had to increase a couple of times, but I saw same overlapping effect in Address Book.

To fix I did either of the following:
Either:

• set - View > Density > Normal
  This instantly fixed the overlapping font without needing to reduce the font size.

OR

• Keep using Density > 'Compact', but reduce the font size :
• View > Font Size > 'Reduce font size' or 'Reset font size'

List of Support Forum reports of issue:
https://support.mozilla.org/en-US/questions/1391942
https://support.mozilla.org/en-US/questions/1398465
https://support.mozilla.org/en-US/questions/1402635