Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

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

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

A <mark is displayed at the left end of the tab bar.

Is it the left scroll button? You may find it's other half -- the > button -- on the other end. Normally the pinned tabs appear to the left of the left scroll button, though, so I'm not sure why yours is after it?

Thank you for the advice. After starting firefox in safe mode, I checked it and the tabs were displayed correctly. I also looked it up and found that Userchrome.css was the cause. I deleted Userchrome.css and it was displayed normally even with a normal reboot. Thank you for your problem-solving advice.

Usually you control the contents of your userChrome.css file, but if you inherited it from someone else, then it probably makes sense to delete it or start from scratch. If it contained important rules you want to continue using, then you might only strip out things that are of uncertain value of unknown provenance.

Thank you very much. I was able to solve it safely. The userChrome.css deleted this time does not contain important rules, so delete the contents and leave it as an empty folder.