Strange white numbers in Thunderbird and Firefox

I first noticed this bug in Thunderbird, when I was writing an email and only the numbers appeared in a strange font and only in white:


Thought this to be a TB specific problem, but by accident I discovered the same thing in uMatrix (extension) in Firefox.

Cannot upload second screenshot :confused:

So it only seems to affect Mozilla programs, in the same extension on a Chromium based browser the numbers were displayed normal.

Any ideas how to fix this?

Operating System: Manjaro Linux
KDE Plasma Version: 5.16.3
KDE Frameworks Version: 5.60.0
Qt Version: 5.13.0
Kernel Version: 5.2.1-1-MANJARO
OS Type: 64-bit
Processors: 4 × Intel® Core™ i7-6600U CPU @ 2.60GHz
Memory: 15,1 GiB of RAM

Firefox screenshot:


Regarding uMatrix, it seems that it's the version number - 1.3.16 is the latest version according to the add-on's page
This might be related to the theme you're using.

As for the numbers in Thunderbird, it's not clear from your picture where exactly they appear in the program's interface - no idea what it might be.


The version number is not the problem, it is the font.
Like I said it does not happen in Chromium based browsers.
The Thunderbird picture is from something I typed in a reply, I highlighted the numbers with the cursoer, otherwise they are not visible.

Hello! Can you do sudo pacman -Syyu

Regarding Firefox and uMatrix:

Is your system is up-to-date?
Have you made some tests with different themes for GTK applications to rule out the possibility that it's a theme issue?
Are you using any other extensions in Firefox that could cause this issue? If yes, disable them and see if the issue persists.

If nothing else works, try this workaround:

  1. Close Firefox and create a new folder called “chrome” in your Firefox profile directory (to find the profile directory select in Firefox: Help > Troubleshooting Information > under Profile Directory click on Open Directory , and then close Firefox).

  2. In the “chrome” folder create a new file called “userContent.css”.

  3. Open the “userContent.css” file and copy/paste the following CSS code in it:

    /* uMatrix */

    div.paneHead a#gotoDashboard span#version {
        font-size: 10px !important;
        font-weight: normal !important;
        color: #bbb !important;
  1. Save the file and start Firefox to see if the text in uMatrix has changed.

Regarding Thunderbird:

Did this issue happen only once or multiple times?
Did you type the highlighted text or did you copy/paste it from another application to the message pane? If you copied/pasted it there is a chance that the initial format of the text was preserved in Thunderbird.
Are you using any extensions in Thunderbird. If yes disable them or start Thunderbird in safe mode (thunderbird --safe-mode) to see if the issue persists.

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