I’m not aware of an addon that will do this. There were some for earlier version of Thunderbird (I’m assuming you have 68 or higher) but they no longer work.
One relatively easy way - but not quite what you asked for - is to increase the size of the default font everywhere. This is done by changing a preference: layout.css.devPixelsPerPx
See here for more detail: https://support.mozilla.org/en-US/questions/1268779
It is possible to change the font size in the folder and thread panes using a rather technical method called ‘userChrome.css’.
I’ll describe very briefly what you have to do you can decide whether it’s within your capabilities:
1 Locate your Thunderbird profile folder
2 Create a folder ‘chrome’
3 In that folder create a text file ‘userChrome.css’ (case sensitive!) containing this:
#folderTree {
font-size: 16px !important;
}
#threadTree{
font-size: 16px !important;
}
Change the ‘16’ to whatever suits you.
4 If (or when) you have TB version 78 you have to enable this by setting the preference toolkit.legacyUserProfileCustomizations.stylesheets to TRUE
You’ll find more information about userChrome stylesheets, and how to locate your profile here: http://kb.mozillazine.org/UserChrome.css
userChrome.css stylesheets are a ‘legacy’ technology which has been in Firefox and Thunderbird for years. It may be removed at some stage - but my guess is it’ll be good for a few years yet!