Firefox displays left and right scroll buttons on the tab bar when there are too many tabs to display. Are those the ones you're seeing, or different arrows?

No. These are large arrows (each contained in a circle about 1 inch in diameter) that appear about halfway down on the left (for back) and the right (for forward) of the window. When triggered, they rapidly move from either edge of the window to about 2 inches in and then they jump back out (usually after doing their damage). The back arrow appears more frequently.

Start Firefox in Troubleshoot 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 Troubleshoot Mode start window

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