It looks like the Bookmarks/History Sidebar is displayed at the bottom of your screen. I’m not sure how you managed to achieve that, but if it is, pressing CTRL+B or unchecking View > Sidebars > Bookmarks/History should do the trick.

Correction: it looks like the "Show all Bookmarks" window that you might be able to switch off using Ctrl+Shift+B or using Bookmarks menu - Show all Bookmarks. Nevertheless, does this also happen in Safe mode? If not, some add-on may cause this uncommon display behaviour.

Hi Tonnes, many thanks for help and the tip. Bookmarks Sidebar changing on/off with Ctrl+Shift+B, but not the disturbing bottom window.

In safe mode, there is NO disturbing window - great. What does this tell you?

Good, could you disable your add-ons either one by one (or all, and enable them one by one) as described here to find one that is probably causing the issue?

Start Firefox in Safe Mode to check if one of the extensions (3-bar Menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: 3-bar Menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes