No one has an idea ? I rather not revert back to an older version :(

It seems the text is too large for the space allowed (or the space is too small for the text).

Do you use any custom style rules (user styles) or user scripts that modify the YouTube layout? If so, please try disabling them temporarily (sorry if that leads to temporary blindness due to a bright white background) and see whether that has any effect.

Minimum font size

If your Firefox has a large "minimum font size" setting, text may exceed the space allotted. That would be the next thing to check:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

Scroll down to Language & Appearance, Fonts & Colors, and click the Advanced button. At the top, Firefox will show a character set. Most like, the "Latin" settings are used by YouTube. Make sure the "Minimum font size" is no larger than 9. Then click OK to save the change.

Instead of using a minimum font size, you can enlarge text and images proportionally using zoom.

If image sizes do not change when you zoom, you may have "Zoom Text Only" set. To check that, activate the menu bar (e.g., tap the Alt key or F10 key) and open View > Zoom. If there is a checkmark for Zoom Text Only, click that to "uncheck" it.

Thanks for your reply. The "zoom text only" solved the problem. Thanks again !