Type about:preferences#general<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Make sure you haven't enabled a High Contrast theme in the OS settings.

Hello FredMcD Thanks for your help but everything was already sorted out as suggested. And I still have the same annoyance. Maybe another idea?

It might be useful to have the configuration of my PC.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem,

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

While you are in Troubleshoot(Safe) mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Hello Jacques, is the example screenshot from Google? Usually Google prefers Arial font, and for the bolded words, Arial Bold. However, each Linux distribution may have different fonts, so it would be helpful if you could poke around a bit to discover what font is being used for the problem text. Here's how:

right-click the area with the incorrect text, then click "Examiner l'élément" (or in English, Inspect Element)

Firefox should open a Developer Tools panel and show the Page Inspector panel, which the right-clicked element highlighted in a tree-style diagram of the page's source.

On the right side are one or two panels with information about the highlighted element. The right-most panel can be switched to show Font information ("Polices"). Sometimes you need to click the tiny triangle at the right end of the panel headings to switch to that one, depending on the width.

This panel shows the actual font being used by Firefox for that text:

What does yours show?

Hello Jacques, your replies haven't appeared here yet because they contained a link to a non-Mozilla website. They are in the spam review queue and should be made visible soon.

Meanwhile, in case there is a problem with your Arial fonts, could you compare this thread: https://support.mozilla.org/questions/1344141