I have "when I open a link in a new tab, switch to it immediately" unchecked, but it always switches to the tab anyway. Any ideas on how to fix this?

This issue can be caused by an extension.

Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

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

Thanks for our reply. I reset Firefox and I still have the same problem.

How are you opening the link: just clicking it, or specifically targeting a new tab by holding the Ctrl key while clicking, or using the "middle" mouse button? I think on a plain click, Firefox will follow the link, but with the other two, you will stay on the same page.