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 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

Boot the computer in Windows Safe mode with network support to see if that has effect in case other software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

I have the same problem, but only on GoComics.com on the Comics I Follow page. Clicking with any mouse button sends that same tab to that comic's page, along with whatever the middle and right buttons should do.

Left-clicking on a comic should, and does, go to that comic's page in the same tab. Middle-clicking on it should go to that comic's page in a new tab, which it does, but the original tab also goes to that comic's page. Right-clicking should bring up a menu, which it does, but again, underneath the popup menu, the page goes to that comic's page.

This just started a few months ago. The GoComics pages work correctly using Edge and Chrome, but I prefer Firefox.