In Windows 10, I use Thunderbird 78.11.0 (32-Bit) as an e-mail client.

Today, suddenly the font size of the message list and of the folder list has become very small. (No changes in other applications).

How can I adjust the font size of the message list and of the folder list in Thunderbird?

  • Have you looked at Windows 10 scaling and text size. Settings, System and then in Settings Search type in text size, or change size of text. Make text bigger. I do both of the above.
    – John
    Jul 4, 2021 at 13:05
  • Scaling and text size is OK for all my other apps. It's just Thunderbird. Jul 4, 2021 at 13:07
  • I do not use Thunderbird, just Outlook, so I cannot assist with Thunderbird.
    – John
    Jul 4, 2021 at 13:08

4 Answers 4


Open the Menu and go to Preferences > General. Scroll all the way down and at the very bottom, click the Config Editor... button (accept the warning).

You're looking for layout.css.devPixelsPerPx, you can just search 'pixels' and it'll show up.

Click on layout.css.devPixelsPerPx and edit the default value of -1.0 to a scale factor you are comfortable with (example: 1.5, 2.0, 2.25, etc).

Clicking OK will allow you to immediately see your changes so you can dial it in pretty easily.

  • FANTASTIC - Thank you! A value of 1.3 made it just right. But now the LINE-HEIGHT in the Message List and in the Folder List is still too big. How can I decrease the line-height? Jul 4, 2021 at 15:22
  • If an answer has helped you solve your problem feel free to mark the answer as accepted. Jul 5, 2021 at 9:53
  • I have accepted the answer before, but mysteriously the accept check had disappeared. So now I accepted it again. Jul 5, 2021 at 10:51
  • The scale factor is the only size adjustment I’m aware of without going in and editing the CSS or something. Unfortunately I can’t really help with that part.
    – kyer
    Jul 5, 2021 at 12:30
  • 1
    This is not a solution, this increases the size of everything. Sep 25, 2021 at 10:16

This is the actual solution to your question. This was given in the Thunderbird support forum so it's not my production.

Try this css code:

/* Threads Pane font and background*/
#threadTree > treechildren {
  font-size: 12pt !important;
  font-family: Times !important;
  background-color: lightgrey !important;
  color: navy !important;

/* Folder Pane font and background*/
#folderTree > treechildren {
  font-size: 12pt !important;
  font-family: Times !important;
  background-color: lightgrey !important;
  color: navy !important;

Help/Troubleshooting, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files ., copy in the above code, change the numbers and colors as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Options(Preferences)/General/Config. editor, restart TB.

  • This is the correct answer – the accepted answer will scale the body of the emails but not the font in the folder-list or message-list (actually, the only relevant lines within the CSS are the "font-size", font-family and colors are another matter)
    – Dweia
    Jan 18 at 9:05
  • Like the accepted solution, it's a partial solution. If you go too big with this, you lose sync with the gray bars. May 4 at 20:13
  • This and the accepted solution complement each other well. Note that font-family can be whatever suits your taste (try Baskerville, Garamond, or Caslon, if available), but it is (at least running 102 on an M2 Mac with Ventura) completely ignored in the "folderTree" section (that whole section can be safely omitted). Also, "background-color" and "color" clauses can be safely omitted (they made it look terrible on my new Mac). May 4 at 20:23

I spent a couple of hours today trying to make userChrome.css work with Thunderbird 91 on Linux Mint. No matter what I did the file was ignored. I finally found the answer: go into the config editor and search for: toolkit.legacyUserProfileCustomizations.stylesheets and set that to true. After doing that, the userChrome.css file works as expected.


layout.css.devPixelsPerPx does not scale the fonts for me and I could not get it get userChrome.css to load.

What works like a charm is to set the environment variable GDK_DPI_SCALE


$ GDK_DPI_SCALE=2 thunderbird

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