Comment # 10 on bug 1132954 from Alexander Shchadilov

I can reproduce font shrinking in Thunderbird with Leap 15.2, however I found a
workaround for it. For some reason fonts do not fit in rows of lists (account
folders, letters), so increasing row height via css file improves the
situation.
Thunderbird interface can be tweaked with a userChrome.css file that does not
exist by default. It should be created in 
~/.thunderbird/<your-thunderbird-profile>/chrome/
where <your-thunderbird-profile> is a folder with an alphanumeric name
generated by Thunderbird.
Code to put into userChrome.css:

treechildren::-moz-tree-row {
    margin-top: 2px !important;
    margin-bottom: 2px !important;
}

treechildren::-moz-tree-row(selected) {
    margin-top: 0px !important;
    margin-bottom: 0px !important;
}

As for tincts, I believe that you noticed a sub-pixel anti-aliasing and it is
completely irrelevant to this Thunderbird behaviour. You should be able to
change font rendering to grayscale in XFCE settings (Appearance, Fonts, set
Sub-pixel order to "None").