Next time you see this, could you right-click the Back button and see how much unexpected history appears between the current page and Yahoo? There are some news sites that add history entries as you scroll up and down, and I don't remember if there is a way to prevent that. But using the right-click menu of the Back button, you should be able to can skip over the extra entries and go back to where you want.

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 -> Appearance
• 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

You can consider uBlock Origin as a replacement for Adblock Plus to see if that works better (lower memory footprint).

• https://addons.mozilla.org/firefox/addon/ublock-origin/