u/asarch

I am using Slackware 15-stable with Firefox from the web site and this is its appearance:

https://preview.redd.it/0yjavetr2vyg1.png?width=1920&format=png&auto=webp&s=af13448a6f70f13ef9659e66c684d41d3ebcd8ae

Even other Mozilla products like Thunderbird (also from its web site) have the same problem, what could be wrong?