Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Mozilla Thunderbird's message list, mailbox list, and headers use a font size that is so large, I can barely see any content in the message preview pane. I'd like to reduce these to 10px, and reduce the headers to 8px or less. How can I do this?

share|improve this question
up vote 3 down vote accepted

Thunderbird is written is XUL. It's Mozilla's markup language, and it's powered by XULRunner. Basically, it's GUI-oriented XML.

The thing that styles the whole application is actually just a simple .css file. If you find it, you can then find the elements you are looking for and just tweak the CSS. I'll post back the required path to the file and the rules to be tweaked.

share|improve this answer
"you can then find the elements you are looking for and just tweak the CSS". I'm a website developer so tweaking CSS is great. I just need to know the selectors for the parts of the UI. – Josh Dec 13 '10 at 15:32
Heh, nice. Basically, the layout is simple: all of the program's guts are in chrome/, inside of JAR files. I haven't tweaked Firefox or Thunderbird, but if you install Firebug or a similar extension, you can get the class of the rows. – Blender Dec 13 '10 at 15:40
I found something that seems plausible: chrome/classic.jar/skin/classic/messenger/mailWindow1.css. Mozilla was nice and gave the elements really obvious names. – Blender Dec 13 '10 at 15:43
Perfect, thanks Blender! I extracted that .jar into my home directory and was able to learn the rules I needed -- see my answer. – Josh Dec 13 '10 at 20:57
XUL hacking is fun, and really simple. That's how easy it is to make a Firefox extension! – Blender Dec 13 '10 at 22:04

Blender's answer pointed me in the right direction. I didn't actually modify those files, but what I did instead was created a file ~/.mozilla-thunderbird/iddbnhwr.default/chrome/userChrome.css and I put my changes in there. I made mine look like this:

#threadTree {
    font-family: Verdana, Arial, Calibri !important;
    font-size: 10px !important;

#msgHeaderView {
    font-family: Verdana, Arial, Calibri !important;
    font-size: 10px !important;
    height: 100px !important;
    overflow: auto !important;

#folderTree {
    font-family: Verdana, Arial, Calibri !important;
    font-size: 10px !important;

Analyzing the files from Blender's answer showed me that the following are the CSS selectors I wanted:

  • #folderTree - The list of folders on the left hand side
  • #threadTree - The list of messages on the top right.
  • #msgHeaderView - The header pane at the top of every message preview / viewer window

There's a lot more interesting stuff in those files:

  • #mailContent - Looks like the body of mail messages?
  • #folderUnreadCol, #folderTotalCol, #folderSizeCol, #folderNameCol - Self explanatory
  • treecol.flagColumnHeader - Looks like you could change the flag icon to something else... Maybe an upvote icon? ;-)
  • treecol.junkStatusHeader - Same for junk icon. Just change the list-style-image: url(...) rule.
share|improve this answer
Wow, I completely forgot about those user CSS files. I always loved Mozilla for their code. – Blender Dec 14 '10 at 17:12

Generally, you can customise the Thunderbird’s look with the files chrome/userChrome.css (for the UI) and chrome/userContent.css (for the message display), both located in your Thunderbird profile folder. (You might have to create them.)

In order to find the relevant CSS selector, a DOM inspector might help.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.