Whoops, this can happen when Firefox is asked to open some kind of content it can't display. There is an old article on that: Firefox repeatedly opens empty tabs or windows after you click on a link.

If you want to close all of the tabs to the right of the current tab, try this:

right-click the current tab > Close Multiple Tabs > Close Tabs to Right

There also are options to close to the left or close all of the other tabs. Whatever seems best to you.

Unfortunately, I can't think of a way to close just the blank tabs, but hopefully they are all to one side or the other.

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