After the system crash, the default font in the folder list and all menus in Thunderbird suddenly changed to bold (screenshot #1). To check what is happening, I downloaded Theme Font Size Changer plugin. Now I see that in this plugin, the "Default" font is shown in bold regardless of the setting of the "Weight" drop-down box (see the attached screenshot #2 to see what I mean). After looking further, I saw that this new default font is actually "Tahoma bold", and, if I look at the Tahoma font in the font drop-down menu in this plugin, I can see that only bold font is shown there (see screenshot #3). The default font for all these menus used to be Tahoma plain, but this font is not visible in Thunderbird anymore. I can select other fonts in this plugin, for them, everything works as before, i.e. the plain/bold fonts are used where necessary, but I got used to Tahoma plain in Thunderbird menus and lists and changing it to something else is inconvenient. What is more strange, is that the fonts in Firefox did not change, and when I installed the same plugin there, it shows the default font as plain Tahoma correctly (see screenshot #4), so it looks like it is not a system font issue. Any help is appreciated.