Thunderbird (and Firefox) plain text font changed to small bold after update and cannot be changed

After updating today to latest packages the fonts used to display and compose plain text messages in Thunderbird has changed. Formerly the fonts were well readable normal sans fonts, but today they changed to a smaller and bold, quite unreadable font.
Here is the original, good looking font:

And this is the ugly new font:

The screenshots were created with the same zoom level. All fonts in plain text messages, and plain text compose windows are like this. Also in Firefox directory listings have the same issue.

I have tried to change fonts in Tb, but nothing has changed, regardless which font I have selected.

What happened? How could I change back? Maybe it is a KDE font issue?

Check the GTK fonts in Systemsettings - Desktop Style.

From the factory mailing list:

Firefox: weird mono fonts since yesterday - (which I’m guessing may also apply to Thunderbird)

suggests it may be this bug:

Thanks for the advises.

It is definitely the texlive-sourcecodepro related bugaffecting monospace font. After removing texlive-sourcecodepro* packages the good old font is back again.

Thanks, worked for me.