All, One giant headache moving to Tbird 91 (aside from the loss of enigmail and the screwy way gpg keys are handled) is that font sizing just sucks. Nothing works to set the folder pane (tree) or thread pane (tree) to something readable. The new approach is to have you use the aboud:config editor and set: layout.css.devPixelsPerPx ( 1.0 is base, can go in increments of .05, etc.. to grow or shrink interface font size) However, in reality all this does is stretch the line height and there is no independent control of the folder pane and thread pane. So if you want to increase the size of the message list (thread pane) you end up making the folder list too big and vice versa. The problem is Mozilla, in true Plasma or Gtk4 fashion has removed the ability to set the interface elements through userChrome. The did so by disabling the following by default: toolkit.legacyUserProfileCustomizations.stylesheets The good news is you can set that to true and have the userChrome file read by Tbird on start. This will restore the ability to adjust the font size of the folder pane or thread pane individually, e.g. /* thread pane font */ #threadTree > treechildren { font-size: 9pt !important; font-family: sans-serif !important; } /* folder pane font */ #folderTree > treechildren { font-size: 8pt !important; font-family: sans-serif !important; } After wasting an hour playing with various approaches I stumbled onto the following answer on Unix & Linux on StackExchange: https://superuser.com/a/1678336/439609 Hopefully this will save someone else the wasted hour trying to figure out why what has always worked for fonts no longer works in Tbird 91 -- David C. Rankin, J.D.,P.E.